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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?
>
> > >
> > >>
> > >> >> >
> > >> >> >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=lDHJ2FW52oJ3l
> > >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=lDHJ2FW52oJ3lq
> > >q
> > >> >sArgFRdcevq01tbLQAw4A_NO7xgI&m=gIIx9_eEGj-aDaM6Z-
> > >>
> > >>42yWtI9MnZcqZ2Gw7KCN7EgCg&s=4e2kWmSdPAtGMXBTHwfgNE_KOZdDU
> > >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=lDHJ2FW52oJ3lqqsArgF
> > >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=nKjWec2b6R0mOyPaz7xtfQ
> > >&
> > >> >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=lDHJ2FW52
> > >o
> > >> >J3lqqsArgFRdcevq01tbLQAw4A_NO7xgI&m=gIIx9_eEGj-aDaM6Z-
> > >>
> > >>42yWtI9MnZcqZ2Gw7KCN7EgCg&s=cKbLeI_5Fu9G7aybE5u51yISB0eRer6BvC
> > >xr
> > >> >wgd5HS4&e=
> > >>
> >
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