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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: 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.
>
>>
>> >> >
>> >> >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}
>> >
>> >
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>> >
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>&
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