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Subject: RE: [virtio-comment] Re: [EXT] Re: [virtio-comment] Re: [PATCH] virtio-net: Add equal-sized receive buffers feature
>-----Original Message-----
>From: Michael S. Tsirkin <mst@redhat.com>
>Sent: Sunday, 1 December, 2019 23:44
>To: Vitaly Mireyno <vmireyno@marvell.com>
>Cc: Jason Wang <jasowang@redhat.com>; virtio-comment@lists.oasis-
>open.org
>Subject: Re: [virtio-comment] Re: [EXT] Re: [virtio-comment] Re: [PATCH]
>virtio-net: Add equal-sized receive buffers feature
>
>On Sun, Dec 01, 2019 at 08:07:55AM -0500, Michael S. Tsirkin wrote:
>> On Sun, Dec 01, 2019 at 10:22:17AM +0000, Vitaly Mireyno wrote:
>> >
>> >
>> > >-----Original Message-----
>> > >From: Michael S. Tsirkin <mst@redhat.com>
>> > >Sent: Wednesday, 27 November, 2019 15:51
>> > >To: Vitaly Mireyno <vmireyno@marvell.com>
>> > >Cc: Jason Wang <jasowang@redhat.com>; virtio-comment@lists.oasis-
>> > >open.org
>> > >Subject: Re: [virtio-comment] Re: [EXT] Re: [virtio-comment] Re:
>> > >[PATCH]
>> > >virtio-net: Add equal-sized receive buffers feature
>> > >
>> > >On Tue, Nov 26, 2019 at 05:27:31PM +0000, Vitaly Mireyno wrote:
>> > >>
>> > >>
>> > >> >-----Original Message-----
>> > >> >From: virtio-comment@lists.oasis-open.org
>> > >> ><virtio-comment@lists.oasis- open.org> On Behalf Of Michael S.
>> > >> >Tsirkin
>> > >> >Sent: Sunday, 24 November, 2019 17:30
>> > >> >To: Vitaly Mireyno <vmireyno@marvell.com>
>> > >> >Cc: Jason Wang <jasowang@redhat.com>;
>> > >> >virtio-comment@lists.oasis- open.org
>> > >> >Subject: [virtio-comment] Re: [EXT] Re: [virtio-comment] Re:
>> > >> >[PATCH]
>> > >> >virtio-
>> > >> >net: Add equal-sized receive buffers feature
>> > >> >
>> > >> >On Sun, Nov 24, 2019 at 03:02:05PM +0000, Vitaly Mireyno wrote:
>> > >> >>
>> > >> >>
>> > >> >> >-----Original Message-----
>> > >> >> >From: Michael S. Tsirkin <mst@redhat.com>
>> > >> >> >Sent: Wednesday, 20 November, 2019 15:23
>> > >> >> >To: Vitaly Mireyno <vmireyno@marvell.com>
>> > >> >> >Cc: Jason Wang <jasowang@redhat.com>;
>> > >> >> >virtio-comment@lists.oasis- open.org
>> > >> >> >Subject: Re: [EXT] Re: [virtio-comment] Re: [PATCH] virtio-net:
>> > >> >> >Add
>> > >> >> >equal- sized receive buffers feature
>> > >> >> >
>> > >> >> >On Thu, Nov 14, 2019 at 03:49:59PM +0000, Vitaly Mireyno wrote:
>> > >> >> >>
>> > >> >> >>
>> > >> >> >> >-----Original Message-----
>> > >> >> >> >From: Michael S. Tsirkin <mst@redhat.com>
>> > >> >> >> >Sent: Monday, 11 November, 2019 17:05
>> > >> >> >> >To: Vitaly Mireyno <vmireyno@marvell.com>
>> > >> >> >> >Cc: Jason Wang <jasowang@redhat.com>;
>> > >> >> >> >virtio-comment@lists.oasis- open.org
>> > >> >> >> >Subject: Re: [EXT] Re: [virtio-comment] Re: [PATCH] virtio-net:
>> > >> >> >> >Add
>> > >> >> >> >equal- sized receive buffers feature
>> > >> >> >> >
>> > >> >> >> >On Mon, Nov 11, 2019 at 01:58:51PM +0000, Vitaly Mireyno
>wrote:
>> > >> >> >> >>
>> > >> >> >> >>
>> > >> >> >> >> >-----Original Message-----
>> > >> >> >> >> >From: Michael S. Tsirkin <mst@redhat.com>
>> > >> >> >> >> >Sent: Sunday, 10 November, 2019 17:35
>> > >> >> >> >> >To: Vitaly Mireyno <vmireyno@marvell.com>
>> > >> >> >> >> >Cc: Jason Wang <jasowang@redhat.com>;
>> > >> >> >> >> >virtio-comment@lists.oasis- open.org
>> > >> >> >> >> >Subject: Re: [EXT] Re: [virtio-comment] Re: [PATCH] virtio-
>net:
>> > >> >> >> >> >Add
>> > >> >> >> >> >equal- sized receive buffers feature
>> > >> >> >> >> >
>> > >> >> >> >> >On Sun, Nov 10, 2019 at 02:13:14PM +0000, Vitaly Mireyno
>wrote:
>> > >> >> >> >> >>
>> > >> >> >> >> >> >-----Original Message-----
>> > >> >> >> >> >> >From: virtio-comment@lists.oasis-open.org
>> > >> >> >> >> >> ><virtio-comment@lists.oasis- open.org> On Behalf Of
>> > >> >> >> >> >> >Michael
>> > >S.
>> > >> >> >> >> >> >Tsirkin
>> > >> >> >> >> >> >Sent: Tuesday, 5 November, 2019 20:52
>> > >> >> >> >> >> >To: Jason Wang <jasowang@redhat.com>
>> > >> >> >> >> >> >Cc: Vitaly Mireyno <vmireyno@marvell.com>;
>> > >> >> >> >> >> >virtio-comment@lists.oasis- open.org
>> > >> >> >> >> >> >Subject: [EXT] Re: [virtio-comment] Re: [PATCH] virtio-
>net:
>> > >> >> >> >> >> >Add equal-sized receive buffers feature
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >External Email
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >----------------------------------------------------
>> > >> >> >> >> >> >-----
>> > >> >> >> >> >> >---
>> > >> >> >> >> >> >---
>> > >> >> >> >> >> >---
>> > >> >> >> >> >> >---
>> > >> >> >> >> >> >- On Fri, Nov 01, 2019 at 03:22:26PM +0800, Jason Wang
>wrote:
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >> On 2019/11/1 äå12:09, Michael S. Tsirkin wrote:
>> > >> >> >> >> >> >> > On Thu, Oct 31, 2019 at 10:46:55AM +0000, Vitaly
>> > >> >> >> >> >> >> > Mireyno
>> > >> >wrote:
>> > >> >> >> >> >> >> > > The feature is limited to receive buffers only.
>> > >> >> >> >> >> >> > > A driver decides on receive buffer length. The
>> > >> >> >> >> >> >> > > only limitation is that this
>> > >> >> >> >> >> >length has to be the same for all receive virtqueue
>buffers.
>> > >> >> >> >> >> >> > > The driver configures receive buffer length to
>> > >> >> >> >> >> >> > > the device during device
>> > >> >> >> >> >> >initialization, and the device reads it and may use
>> > >> >> >> >> >> >it for optimal
>> > >> >> >> >operation.
>> > >> >> >> >> >> >> > > No changes for transmit buffers.
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > > -----Original Message-----
>> > >> >> >> >> >> >> > > From: virtio-comment@lists.oasis-open.org
>> > >> >> >> >> >> >> > > <virtio-comment@lists.oasis-open.org> On
>> > >> >> >> >> >> >> > > Behalf Of Jason Wang
>> > >> >> >> >> >> >> > > Sent: Thursday, 31 October, 2019 12:15
>> > >> >> >> >> >> >> > > To: Vitaly Mireyno <vmireyno@marvell.com>;
>> > >> >> >> >> >> >> > > virtio-comment@lists.oasis-open.org
>> > >> >> >> >> >> >> > > Cc: Michael S. Tsirkin <mst@redhat.com>
>> > >> >> >> >> >> >> > > Subject: [virtio-comment] Re: [PATCH] virtio-net:
>> > >> >> >> >> >> >> > > Add equal-sized receive buffers feature
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > > On 2019/10/31 äå5:23, Vitaly Mireyno wrote:
>> > >> >> >> >> >> >> > > > Some devices benefit from working with
>> > >> >> >> >> >> >> > > > receive buffers of a
>> > >> >> >> >> >> >predefined constant length.
>> > >> >> >> >> >> >> > > > Add a feature for drivers that allocate
>> > >> >> >> >> >> >> > > > equal-sized receive buffers, and
>> > >> >> >> >> >> >devices that benefit from predefined receive buffers
>length.
>> > >> >> >> >> >> >> > > >
>> > >> >> >> >> >> >> > > > Signed-off-by: Vitaly Mireyno
>> > >> >> >> >> >> >> > > > <vmireyno@marvell.com>
>> > >> >> >> >> >> >> > > > ---
>> > >> >> >> >> >> >> > > > content.tex | 29
>+++++++++++++++++++++++++++--
>> > >> >> >> >> >> >> > > > 1 file changed, 27 insertions(+), 2
>> > >> >> >> >> >> >> > > > deletions(-)
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > > Is there any other networking device that has
>> > >> >> >> >> >> >> > > this
>> > >feature?
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > > > diff --git a/content.tex b/content.tex index
>> > >> >> >> >> >> >> > > > b1ea9b9..c9e67c8
>> > >> >> >> >> >> >> > > > 100644
>> > >> >> >> >> >> >> > > > --- a/content.tex
>> > >> >> >> >> >> >> > > > +++ b/content.tex
>> > >> >> >> >> >> >> > > > @@ -2811,6 +2811,10 @@ \subsection{Feature
>> > >> >> >> >> >> >> > > > bits}\label{sec:Device
>> > >> >> >> >> >> >Types / Network Device / Feature bits
>> > >> >> >> >> >> >> > > > \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set
>> > >MAC
>> > >> >> >> >> >> >> > > > address
>> > >> >> >> >> >> >through control
>> > >> >> >> >> >> >> > > > channel.
>> > >> >> >> >> >> >> > > > +\item[VIRTIO_NET_F_CONST_RXBUF_LEN(58)]
>> > >> >> >> >> >> >> > > > +Driver allocates all
>> > >> >> >> >> >> >receive buffers
>> > >> >> >> >> >> >> > > > + of the same constant length. Device
>> > >> >> >> >> >> >> > > > + benefits from working
>> > >> >> >> >with
>> > >> >> >> >> >> >> > > > + receive buffers of equal length.
>> > >> >> >> >> >> >> > > > +
>> > >> >> >> >> >> >> > Problem is, I don't think linux will use this
>> > >> >> >> >> >> >> > for skbs since it breaks buffer accounting. This
>> > >> >> >> >> >> >> > is because it is important to make skbs as small
>> > >> >> >> >> >> >> > as you can. So even if you see "device would
>> > >> >> >> >benefit"
>> > >> >> >> >> >> >> > there is no way to balance this with the benefit to
>linux.
>> > >> >> >> >> >> >> > How do you know which benefit is bigger?
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >> I guess the idea is e.g for Linux driver, it can
>> > >> >> >> >> >> >> refuse this
>> > >feature.
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >Okay. What uses it?
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> > You also never explained how does device
>> > >> >> >> >> >> >> > benefit. My guess is this allows device to
>> > >> >> >> >> >> >> > calculate the # of descriptors to fetch that are needed
>for a packet. Right?
>> > >> >> >> >> >> >> > Assuming this, I think a rough estimate should be
>enough.
>> > >> >> >> >> >> >> > If device fetches too much it can discard extra,
>> > >> >> >> >> >> >> > if it does not fetch enough it can fetch extra.
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> > Let us not forget that express packets are 256
>> > >> >> >> >> >> >> > bytes so up to
>> > >> >> >> >> >> >> > 16 descriptors fit in a packet, there is no
>> > >> >> >> >> >> >> > benefit most of the time in knowing whether e.g.
>> > >> >> >> >> >> >> > 1 or 2 descriptors are
>> > >> >needed.
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> > Let us not forget these are buffers, not descriptors.
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >> I guess maybe the initial motivation is constant
>> > >> >> >> >> >> >> descriptor
>> > >> >length.
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >That conflicts with requirement framing is up to driver.
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >
>> > >> >> >> >> >> I was using the wrong terminology. The feature is
>> > >> >> >> >> >> about constant
>> > >> >> >> >> >*descriptor* length. In other words, the value that
>> > >> >> >> >> >driver puts in Packed Virtqueue "Element Length" field (or
>'len'
>> > >> >> >> >> >field in the
>> > >> >> >'pvirtq_desc'
>> > >> >> >> >struct).
>> > >> >> >> >> >
>> > >> >> >> >> >OK so this conflicts with "2.6.4 Message Framing" which
>> > >> >> >> >> >requires that drivers can split buffers into as many
>> > >> >> >> >> >descriptors as
>> > >> >they like.
>> > >> >> >> >> >Right?
>> > >> >> >> >> >
>> > >> >> >> >> >> Does it make more sense now, or you still see an
>> > >> >> >> >> >> issue with Linux
>> > >> >> >driver?
>> > >> >> >> >> >
>> > >> >> >> >> >I think there's an issue with the flexible framing
>> > >> >> >> >> >requirements and there's an issue with Linux driver.
>> > >> >> >> >> >
>> > >> >> >> >> >> The motivation is to allow the device to calculate
>> > >> >> >> >> >> the exact number of
>> > >> >> >> >> >descriptors to consume, before fetching the descriptors.
>> > >> >> >> >> >This is beneficial for devices for which overconsuming
>> > >> >> >> >> >or underconsuming descriptors come at a high cost.
>> > >> >> >> >> >
>> > >> >> >> >> >So I guess we can agree getting the # of descriptors
>> > >> >> >> >> >*exactly* right isn't all that important. Right? My
>> > >> >> >> >> >question is how does the driver balance the device versus
>Linux needs?
>> > >> >> >> >> >
>> > >> >> >> >> >One idea is if we assume this is best effort, does not
>> > >> >> >> >> >have to be exact, then how about just having the device
>> > >> >> >> >> >assume descriptor sizes stay more or less constant and
>> > >> >> >> >> >use that to estimate the amount? If it under/over
>> > >> >> >> >> >estimates, things just go a bit
>> > >> >slower.
>> > >> >> >> >> >This way driver can adjust the sizes and device will
>> > >> >> >> >> >react automatically, with time.
>> > >> >> >> >> >
>> > >> >> >> >> >
>> > >> >> >> >>
>> > >> >> >> >> I see that "2.6.4 Message Framing" allows a full
>> > >> >> >> >> flexibility of descriptor lengths, and I presume it's
>> > >> >> >> >> applicable to Packet Virtqueues as well, though defined
>> > >> >> >> >> under Split Virtqueues
>> > >section.
>> > >> >> >> >
>> > >> >> >> >That's a good point. Probably makes sense to move it out
>> > >> >> >> >to a common section, right?
>> > >> >> >> >
>> > >> >> >> >> The example
>> > >> >> >> >> talks about transmit virtqueue, and it makes perfect sense.
>> > >> >> >> >> However, wouldn't a typical driver place equal-sized
>> > >> >> >> >> *receive* descriptors
>> > >> >> >> >anyway? So if a device can benefit from it, the driver
>> > >> >> >> >might as well negotiate this new feature.
>> > >> >> >> >
>> > >> >> >> >Having buffer size jump around wildly doesn't seem too
>> > >> >> >> >useful, I
>> > >agree.
>> > >> >> >> >OTOH being able to adjust it gradually has been in the
>> > >> >> >> >past demontrated to help performance measureably.
>> > >> >> >> >
>> > >> >> >> >> This could be especially relevant for devices, for which
>> > >> >> >> >> adjusting the number
>> > >> >> >> >of used descriptors is impractical after descriptors have
>> > >> >> >> >already been
>> > >> >> >fetched.
>> > >> >> >> >> I agree that if this requirement conflicts with specific
>> > >> >> >> >> driver needs, it will not
>> > >> >> >> >be negotiated, and the device will either underperform in
>> > >> >> >> >certain scenarios, or will not come up at all.
>> > >> >> >> >
>> > >> >> >> >Right so that makes it a challenge. Device says it prefers
>> > >> >> >> >fixed buffers. Is that preference more or less important
>> > >> >> >> >than ability to efficiently support workloads such as TCP small
>queues?
>> > >> >> >> >Driver has no idea and I suspect neither does the device.
>> > >> >> >> >So I don't see how will a Linux driver know that it should
>> > >> >> >> >enable this, neither how will device know it's okay to
>> > >> >> >> >just refuse
>> > >features_ok.
>> > >> >> >> >
>> > >> >> >> >
>> > >> >> >> >On the other hand, current drivers already have logic that
>> > >> >> >> >tries to change buffer sizes in a smooth way. So simply
>> > >> >> >> >caching the last buffer size and assuming all the others
>> > >> >> >> >will be exactly the same will go a long way towards
>> > >> >> >> >limiting how much does device need to
>> > >> >fetch.
>> > >> >> >> >This does imply extra logic on the device to recover if
>> > >> >> >> >things change and the first read did not fetch enough
>> > >> >> >> >buffers, but then it would be required anyway if as you
>> > >> >> >> >say above the failure is not
>> > >> >catastrophic.
>> > >> >> >> >
>> > >> >> >> >
>> > >> >> >> >The feature thus only seems useful for small,
>> > >> >> >> >feature-restrained devices
>> > >> >> >> >- which are prepared to sacrifice some performance to cut
>> > >> >> >> >costs, and which can't recover at all. Is this what you are trying
>to do?
>> > >> >> >> >
>> > >> >> >>
>> > >> >> >> The intention is to enable devices with this specific
>> > >> >> >> limitation to offer virtio offload. The feature can be
>> > >> >> >> redefined such that it would only be offered by devices
>> > >> >> >> that are unable to handle dynamically changing descriptor
>lengths. How does that sound?
>> > >> >> >
>> > >> >> >So it makes more sense when mergeable buffers are disabled
>> > >> >> >(since then buffers are practically all same size).
>> > >> >> >
>> > >> >>
>> > >> >> Actually, mergeable buffers are not a problem. They could be
>> > >> >> enabled, as long as each descriptor is the same length. So
>> > >> >> implementations that prefer optimizing memory utilization over
>> > >> >> jumbo frame performance can negotiate
>VIRTIO_NET_F_MRG_RXBUF
>> > >and allocate smaller buffers.
>> > >> >
>> > >> >So my point was, without VIRTIO_NET_F_MRG_RXBUF, all buffers are
>> > >same
>> > >> >length anyway. So if we are talking about cheap simple hardware,
>> > >> >I guess it can just not have VIRTIO_NET_F_MRG_RXBUF and be done
>> > >> >with
>> > >it?
>> > >> >If device does care about memory utilization then I think it
>> > >> >needs to give driver the flexibility it needs/uses.
>> > >> >No?
>> > >> >
>> > >>
>> > >> It's not a matter of device complexity, but a HW architecture,
>> > >> which could be
>> > >complex, but have this specific limitation.
>> > >
>> > >So - don't do it then?
>> > >
>> > >> I see at least two reasons to support and negotiate
>> > >VIRTIO_NET_F_MRG_RXBUF, while keeping equal-sized descriptor
>limitation:
>> > >> * GRO
>> > >
>> > >You mean LRO?
>> > >
>> > >> * With jumbo packets, if the throughput is capped by the port
>> > >> bandwidth,
>> > >and not by the device/driver per-packet performance, it makes sense
>> > >to optimize memory utilization by allocating small buffers, without
>> > >sacrificing throughput performance.
>> > >
>> > >So we are back to square one, if driver cares about memory
>> > >utilization with 1K packets vs 9K buffers, why not with 100byte
>> > >packets vs 1K buffers? Looks like exactly the same tradeoff.
>> > >
>> > >
>> > >>
>> > >> >Again I can see how we might want to disallow crazy setups with e.g.
>> > >> >1 byte per buffer. That's just abuse, no guest does that anyway.
>> > >> >So asking e.g. for a minimal buffer size sounds very reasonable.
>> > >> >But an option that disables functionality that a popular guest
>> > >> >uses needs a lot of documentation to help device writers figure
>> > >> >out whether they want that option or not, and I'd worry that
>> > >> >even with documentation will
>> > >be misunderstood even if we write it.
>> > >> >When do you enable this?
>> > >> >When you don't care about performance? When you don't care about
>> > >Linux?
>> > >> >
>> > >>
>> > >> I understand that there are guest-side optimizations that require
>> > >> flexibility
>> > >in descriptors length. I can propose the following simple logic:
>> > >> Device - advertise "equal-sized descriptor" feature only if the
>> > >> device is
>> > >absolutely unable to operate otherwise. The device will not set
>> > >FEATURES_OK unless the driver negotiates this feature.
>> > >> Driver - if device advertises "equal-sized descriptor" feature -
>> > >> if possible,
>> > >give up on the flexible descriptors length optimizations. If not -
>> > >give up on the device.
>> > >
>> > >Yes, that's possible. Looks like a rather limited feature, and i'd
>> > >rather we focused on something more widely applicable, but with
>> > >enough disclamers that devices SHOULD NOT set this bit we can maybe
>do that.
>> > >
>> >
>> > I agree that this feature is more of a limitation declaration, rather than an
>enhancement, but let me emphasize that its only purpose is to make more
>*existing* HW devices be virtio compatible.
>> > This feature will allow such HW devices to offer virtio offload with
>VIRTIO_NET_F_MRG_RXBUF capability for GRO/LRO offload.
>> > New device designs should definitely avoid employing this feature.
>>
>> I understand, my question is: can you find a way to make this more
>> generally useful? Something along the lines of the min buffer size
>> suggestion which would avoid creating a special case in the driver?
>> Any idea?
>
>as an example of a vague idea, exposing max rx s/g, min buffer + max buffer
>will allow device to force this from device side.
>
>Is that good? If not, how does driver decide on a good fixed buffer size?
>
Setting max=min=fixed_size by the device will work, but this seems too restrictive, as we still may want to enable the driver to select its buffer size.
I guess driver can select the fixed buffer size based on the MTU.
What if device will request max/min buffer size ratio, and driver will set min buffer size? This can solve the fixed size issue, without forcing a specific size.
Along with the max s/g, maybe it can also help avoiding rx buffer size abuse by the driver (i.e. setting it too low).
>>
>> > >
>> > >>
>> > >> >> >
>> > >> >> >With that in mind, I have an idea: scsi and block already
>> > >> >> >have max sg
>> > >field.
>> > >> >> >How about we add a writeable max sg field, maybe even make it
>> > >> >> >programmable per vq?
>> > >> >> >
>> > >> >> >Thus driver tells device what is it's max s/g value for rx.
>> > >> >> >Worst case device fetches a bit more than it needs.
>> > >> >> >Discarding extra shouldn't be expensive. This looks like it
>> > >> >> >will help even smart devices. What
>> > >> >do you think?
>> > >> >> >
>> > >> >> >This nicely avoids conflicting with the flexible framing requirement.
>> > >> >> >
>> > >> >>
>> > >> >> My intention was to avoid any descriptor length variations,
>> > >> >> for devices that unable fetching or discarding extra
>> > >> >> descriptors. (If in the pipelined HW processing the decision
>> > >> >> on number of descriptors is made in the early stage, and it
>> > >> >> cannot be undone in a later
>> > >stage).
>> > >> >
>> > >> >Frankly discarding unused descriptors looks to me like something
>> > >> >that shouldn't have a high cost in the hardware. I can see how
>> > >> >trying to predict descriptor length, and fetching extra if not
>> > >> >enough was fetched can have a high cost, so to me extensions to
>> > >> >remove the guesswork from the device have value. However a lot
>> > >> >of effort went into trying to reduce e.g. number of pci express
>> > >> >packets needed to fetch
>> > >descriptors.
>> > >> >Each packet fetches 256 bytes anyway, does it not? Not having
>> > >> >an ability to use that information seems like an obvious waste,
>> > >> >and that means ability to keep some fetched descriptors around
>> > >> >even if they are not used for a given packet. Again just my $.02.
>> > >> >
>> > >>
>> > >> In packed virtqueue, discarding unused descriptors (and buffers
>> > >> associated
>> > >with them) can indeed be easy, but reusing them for the next packet
>> > >is complicated (or impossible).
>> > >> I agree that descriptors are being (pre)fetched with the maximum
>> > >> efficiency
>> > >(in terms of PCIe bandwidth), and cached in the device. But the
>> > >decision to fetch is being made according to the number of left
>> > >cached-in descriptors and the expected number of descriptors that will
>be used by the packet.
>> > >> If the expected number of descriptors is larger than the actual
>> > >> one, the next
>> > >fetch decision will be taken too early, and there will be no way to
>> > >reuse excess cached descriptors, and they will have to be discarded.
>> > >
>> > >> Even if it's possible to skip descriptors in the packed virtqueue
>> > >> (is it?), it's
>> > >surely inefficient.
>> > >
>> > >OK I think I understand what you are doing. Device is getting
>> > >buffers 1,2,3 for packet 1, it is meanwhile receiving packet
>> > >2 and decides to get buffers 4,5 for packet 2.
>> > >At this point it finally gets buffers 1-3 and figures out that
>> > >buffers 3 was not needed, but possibly it already started writing packet 2
>into buffers 4.
>> > >What to do about buffers 3 now?
>> > >
>> > >Is above a good example?
>> > >
>> > >If yes then it looks like you are unaware that Descriptors can be
>> > >used out of order, with split or packed ring, with no issues.
>> > >Looks like exactly what you need to address this issue?
>> > >So device will simply proceed with marking buffers 1,2,4,5 as used,
>> > >and store buffers 3 in some kind of internal memory and use it for the
>next packet.
>> > >
>> > >This is exactly the kind of thing out of order was designed for.
>> > >
>> > >Does this answer the question?
>> > >
>> >
>> > The example is good, and the suggested solution is clear. However if we're
>talking about HW device that is designed to process packets/buffers in order,
>this solution could not be applicable.
>>
>>
>>
>> > >>
>> > >> >>
>> > >> >> Defining max s/g sounds like an interesting feature by itself.
>> > >> >
>> > >> >But assuming we have max RX s/g, I guess hardware can set max s/g =
>1?
>> > >> >Then since with !VIRTIO_NET_F_MRG_RXBUF all buffers are forced
>> > >> >to be same length.
>> > >> >
>> > >> >>
>> > >> >> >
>> > >> >> >> >> Could you specify what issue do you see with the Linux
>driver?
>> > >> >> >> >
>> > >> >> >> >See logic around struct ewma.
>> > >> >> >> >
>> > >> >> >> >The first relevant commit is I guess commit
>> > >> >> >> >ab7db91705e95ed1bba1304388936fccfa58c992
>> > >> >> >> > virtio-net: auto-tune mergeable rx buffer size for
>> > >> >> >> >improved performance
>> > >> >> >> >
>> > >> >> >> >this claims a gain of about 10% with large packets which
>> > >> >> >> >isn't earth shattering but also not something we can ignore
>completely.
>> > >> >> >> >And I suspect it can be bigger with smaller packets.
>> > >> >> >> >
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> > So device does not really know the exact # of
>descriptors:
>> > >> >> >> >> >> >> > current drivers do 1 descriptor per buffer but
>> > >> >> >> >> >> >> > nothing prevents more.
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> > Thoughts?
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> > > > \item[VIRTIO_NET_F_RSC_EXT(61)] Device
>> > >> >> >> >> >> >> > > > can process duplicated
>> > >> >> >> >> >> >ACKs
>> > >> >> >> >> >> >> > > > and report number of coalesced
>> > >> >> >> >> >> >> > > > segments and duplicated ACKs @@ -2854,8
>> > >> >> >> >> >> >> > > > +2858,8 @@
>> > >> >> >> >\subsubsection{Legacy Interface:
>> > >> >> >> >> >> >Feature bits}\label{sec:Device Types / Network
>> > >> >> >> >> >> >> > > > \subsection{Device configuration
>> > >> >> >> >> >> >> > > > layout}\label{sec:Device Types /
>> > >> >> >> >> >> >Network Device / Device configuration layout}
>> > >> >> >> >> >> >> > > > \label{sec:Device Types / Block Device /
>> > >> >> >> >> >> >> > > > Feature bits / Device configuration layout}
>> > >> >> >> >> >> >> > > > -Three driver-read-only configuration fields
>> > >> >> >> >> >> >> > > > are
>> > >currently defined.
>> > >> >> >> >> >> >> > > > The \field{mac} address field -always exists
>> > >> >> >> >> >> >> > > > (though is only valid if VIRTIO_NET_F_MAC is
>> > >> >> >> >> >> >> > > > set), and
>> > >> >> >> >> >> >> > > > +The driver-read-only \field{mac} address
>> > >> >> >> >> >> >> > > > +field always exists (though is only valid
>> > >> >> >> >> >> >> > > > +if VIRTIO_NET_F_MAC is set), and
>> > >> >> >> >> >> >> > > > \field{status} only exists if
>> > >> >> >> >> >> >> > > > VIRTIO_NET_F_STATUS is
>> > >set.
>> > >> >> >Two
>> > >> >> >> >> >> >> > > > read-only bits (for the driver) are
>> > >> >> >> >> >> >> > > > currently defined for the status
>> > >> >> >> >> >> >field:
>> > >> >> >> >> >> >> > > > VIRTIO_NET_S_LINK_UP and
>> > >> >VIRTIO_NET_S_ANNOUNCE.
>> > >> >> >> >> >> >> > > > @@ -2875,12 +2879,17 @@ \subsection{Device
>> > >> >> >> >> >> >> > > > configuration
>> > >> >> >> >> >> >layout}\label{sec:Device Types / Network Device
>> > >> >> >> >> >> >> > > > VIRTIO_NET_F_MTU is set. This field
>> > >> >> >> >> >> >> > > > specifies the maximum MTU for
>> > >> >> >> >> >> >the driver to
>> > >> >> >> >> >> >> > > > use.
>> > >> >> >> >> >> >> > > > +The device-read-only field
>> > >> >> >> >> >> >> > > > +\field{rx_buf_len} only exists if
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > > Should be driver-read-only.
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > > > +VIRTIO_NET_F_CONST_RXBUF_LEN is
>negotiated.
>> > >This
>> > >> >> >> >> >> >> > > > +field specifies the receive buffers length.
>> > >> >> >> >> >> >> > > > +
>> > >> >> >> >> >> >> > > > \begin{lstlisting}
>> > >> >> >> >> >> >> > > > struct virtio_net_config {
>> > >> >> >> >> >> >> > > > u8 mac[6];
>> > >> >> >> >> >> >> > > > le16 status;
>> > >> >> >> >> >> >> > > > le16 max_virtqueue_pairs;
>> > >> >> >> >> >> >> > > > le16 mtu;
>> > >> >> >> >> >> >> > > > + le32 rx_buf_len;
>> > >> >> >> >> >> >> > > > };
>> > >> >> >> >> >> >> > > > \end{lstlisting} @@ -2933,6 +2942,13 @@
>> > >> >> >> >> >> >> > > > \subsection{Device configuration
>> > >> >> >> >> >> >> > > > layout}\label{sec:Device Types / Network
>> > >> >Device
>> > >> >> >> >> >> >> > > > A driver SHOULD negotiate the
>> > >> >> >> >> >> >> > > > VIRTIO_NET_F_STANDBY feature if
>> > >> >> >> >> >> >the device offers it.
>> > >> >> >> >> >> >> > > > +A driver SHOULD accept the
>> > >> >> >> >VIRTIO_NET_F_CONST_RXBUF_LEN
>> > >> >> >> >> >> >feature if offered.
>> > >> >> >> >> >> >> > > > +
>> > >> >> >> >> >> >> > > > +If VIRTIO_NET_F_CONST_RXBUF_LEN feature
>has
>> > >been
>> > >> >> >> >> >negotiated,
>> > >> >> >> >> >> >> > > > +the driver MUST set \field{rx_buf_len}.
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > > I think it's device that set the field?
>> > >> >> >> >> >> >> > Makes more sense for the driver, but if you want
>> > >> >> >> >> >> >> > this set
>> > >e.g.
>> > >> >> >> >> >> >> > before buffers are added, you must say so.
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> > > > +
>> > >> >> >> >> >> >> > > > +A driver MUST NOT modify \field{rx_buf_len}
>> > >> >> >> >> >> >> > > > +once it has been
>> > >> >> >> >> >set.
>> > >> >> >> >> >> >> > > > +
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> > This seems very unflexible. I can see how e.g.
>> > >> >> >> >> >> >> > XDP would benefit from big buffers while skbs
>> > >> >> >> >> >> >> > benefit from small
>> > >> >buffers.
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> > This calls for ability to change this.
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >> Yes, but it requires non trivial cleanups for the
>> > >> >> >> >> >> >> old length and place them with new ones.
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >> Thanks
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >Let's see:
>> > >> >> >> >> >> >1 - making buffer smaller: just update config space,
>> > >> >> >> >> >> > then make new buffers smaller
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >2 - making buffers bigger: add larger buffers,
>> > >> >> >> >> >> > once all small ones are consumed update config
>> > >> >> >> >> >> >space
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >2 is tricky I agree. Thoughts?
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >
>> > >> >> >> >> >> Agree. It's doable, provided that the driver will
>> > >> >> >> >> >> follow the update
>> > >> >> >> >> >procedure.
>> > >> >> >> >> >> >
>> > >> >> >> >> >> >>
>> > >> >> >> >> >> >> >
>> > >> >> >> >> >> >> > > > \subsubsection{Legacy Interface: Device
>> > >> >> >> >> >> >> > > > configuration
>> > >> >> >> >> >> >layout}\label{sec:Device Types / Network Device /
>> > >> >> >> >> >> >Device configuration layout / Legacy Interface:
>> > >> >> >> >> >> >Device configuration layout}
>> > >> >> >> >> >> >> > > > \label{sec:Device Types / Block Device /
>> > >> >> >> >> >> >> > > > Feature bits / Device
>> > >> >> >> >> >> >configuration layout / Legacy Interface: Device
>> > >> >> >> >> >> >configuration layout}
>> > >> >> >> >> >> >> > > > When using the legacy interface,
>> > >> >> >> >> >> >> > > > transitional devices and drivers @@
>> > >> >> >> >> >> >> > > > -3241,6 +3257,11 @@ \subsubsection{Setting
>> > >> >> >> >> >> >> > > > Up Receive
>> > >> >> >> >> >> >Buffers}\label{sec:Device Types / Network Devi
>> > >> >> >> >> >> >> > > > If VIRTIO_NET_F_MQ is negotiated, each of
>> > >> >> >> >> >> >> > > > receiveq1\ldots
>> > >> >> >> >> >> >receiveqN
>> > >> >> >> >> >> >> > > > that will be used SHOULD be populated
>> > >> >> >> >> >> >> > > > with receive
>> > >> >> >buffers.
>> > >> >> >> >> >> >> > > > +If VIRTIO_NET_F_CONST_RXBUF_LEN feature
>has
>> > >been
>> > >> >> >> >> >negotiated,
>> > >> >> >> >> >> >> > > > +the driver MUST initialize all receive
>> > >> >> >> >> >> >> > > > +virtqueue descriptors \field{len} field
>> > >> >> >> >> >> >> > > > +with the value configured in
>> > >> >> >> >> >> >> > > > +\field{rx_buf_len} device configuration
>> > >> >> >> >> >> >> > > > +field, and allocate receive
>> > >> >> >> >> >> >buffers accordingly.
>> > >> >> >> >> >> >> > > > +
>> > >> >> >> >> >> >> > > > \devicenormative{\paragraph}{Setting Up
>> > >> >> >> >> >> >> > > > Receive Buffers}{Device Types / Network
>> > >> >> >> >> >> >> > > > Device / Device Operation / Setting Up Receive
>Buffers}
>> > >> >> >> >> >> >> > > > The device MUST set \field{num_buffers}
>> > >> >> >> >> >> >> > > > to the number of descriptors used to @@
>> > >> >> >> >> >> >> > > > -3396,6 +3417,10 @@
>> > >> >> >> >> >> >\subsubsection{Processing of Incoming
>> > >> >> >> >> >> >Packets}\label{sec:Device Types / Network
>> > >> >> >> >> >> >> > > > checksum (in case of multiple
>> > >> >> >> >> >> >> > > > encapsulated protocols, one
>> > >> >> >> >level
>> > >> >> >> >> >> >> > > > of checksums is validated).
>> > >> >> >> >> >> >> > > > +If VIRTIO_NET_F_CONST_RXBUF_LEN has been
>> > >> >> >> >> >> >> > > > +negotiated,
>> > >> >> >> >the
>> > >> >> >> >> >> >device
>> > >> >> >> >> >> >> > > > +MAY use \field{rx_buf_len} as a buffer
>> > >> >> >> >> >> >> > > > +length (instead of reading it from
>> > >> >> >> >> >> >> > > > +virtqueue descriptor
>> > >> >\field{len} field).
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > > Is this safe? What if driver submit a small
>> > >> >> >> >> >> >> > > buffer, then device can read
>> > >> >> >> >> >> >more than what is allowed?
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > > Thanks
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > >
>> > >> >> >> >> >> >> > > > +
>> > >> >> >> >> >> >> > > > \drivernormative{\paragraph}{Processing of
>Incoming
>> > >> >> >> >> >> >> > > > Packets}{Device Types / Network Device /
>> > >> >> >> >> >> >> > > > Device Operation
>> > >> >> >/
>> > >> >> >> >> >> >> > > > Processing of Incoming Packets}
>> > >> >
>> > >> >
>> > >> >This publicly archived list offers a means to provide input to
>> > >> >the OASIS Virtual I/O Device (VIRTIO) TC.
>> > >> >
>> > >> >In order to verify user consent to the Feedback License terms
>> > >> >and to minimize spam in the list archive, subscription is required
>before posting.
>> > >> >
>> > >> >Subscribe: virtio-comment-subscribe@lists.oasis-open.org
>> > >> >Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org
>> > >> >List help: virtio-comment-help@lists.oasis-open.org
>> > >> >List archive: https://urldefense.proofpoint.com/v2/url?u=https-
>> > >> >3A__lists.oasis-2Dopen.org_archives_virtio-
>> > >>
>> >
>>>2Dcomment_&d=DwIFaQ&c=nKjWec2b6R0mOyPaz7xtfQ&r=lDHJ2FW52oJ3
>l
>> > >q
>> > >> >qsArgFRdcevq01tbLQAw4A_NO7xgI&m=gIIx9_eEGj-aDaM6Z-
>> > >> >42yWtI9MnZcqZ2Gw7KCN7EgCg&s=-
>> > >> >JICLquqUnNye7tilUS67AFv7opngsKEl5L75acB64U&e=
>> > >> >Feedback License:
>> > >> >https://urldefense.proofpoint.com/v2/url?u=https-
>> > >> >3A__www.oasis-2Dopen.org_who_ipr_feedback-
>> > >>
>> >
>>>5Flicense.pdf&d=DwIFaQ&c=nKjWec2b6R0mOyPaz7xtfQ&r=lDHJ2FW52oJ3l
>q
>> > >q
>> > >> >sArgFRdcevq01tbLQAw4A_NO7xgI&m=gIIx9_eEGj-aDaM6Z-
>> > >>
>> >
>>>42yWtI9MnZcqZ2Gw7KCN7EgCg&s=4e2kWmSdPAtGMXBTHwfgNE_KOZdD
>U
>> > >R
>> > >> >Wsji73HWdVF3A&e=
>> > >> >List Guidelines:
>> > >> >https://urldefense.proofpoint.com/v2/url?u=https-
>> > >> >3A__www.oasis-2Dopen.org_policies-2Dguidelines_mailing-
>> > >>
>> >
>>>2Dlists&d=DwIFaQ&c=nKjWec2b6R0mOyPaz7xtfQ&r=lDHJ2FW52oJ3lqqsAr
>gF
>> > >R
>> > >> >dcevq01tbLQAw4A_NO7xgI&m=gIIx9_eEGj-aDaM6Z-
>> > >> >42yWtI9MnZcqZ2Gw7KCN7EgCg&s=i-dQn5G-
>> > >> >auoFtCxN8Y2PN8UccM1ezgcrsT2A8T1H8wE&e=
>> > >> >Committee: https://urldefense.proofpoint.com/v2/url?u=https-
>> > >> >3A__www.oasis-
>> > >>
>> >
>>>2Dopen.org_committees_virtio_&d=DwIFaQ&c=nKjWec2b6R0mOyPaz7xtf
>Q
>> > >&
>> > >>
>>r=lDHJ2FW52oJ3lqqsArgFRdcevq01tbLQAw4A_NO7xgI&m=gIIx9_eEGj-
>> > >> >aDaM6Z-42yWtI9MnZcqZ2Gw7KCN7EgCg&s=kcv-jA-_-JC3v64-_r5iP-
>> > >> >XQ9rhyaeOKvrHJNHwZrLc&e=
>> > >> >Join OASIS: https://urldefense.proofpoint.com/v2/url?u=https-
>> > >> >3A__www.oasis-
>> > >>
>> >
>>>2Dopen.org_join_&d=DwIFaQ&c=nKjWec2b6R0mOyPaz7xtfQ&r=lDHJ2FW5
>2
>> > >o
>> > >> >J3lqqsArgFRdcevq01tbLQAw4A_NO7xgI&m=gIIx9_eEGj-aDaM6Z-
>> > >>
>> >
>>>42yWtI9MnZcqZ2Gw7KCN7EgCg&s=cKbLeI_5Fu9G7aybE5u51yISB0eRer6Bv
>C
>> > >xr
>> > >> >wgd5HS4&e=
>> > >>
>> >
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