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Subject: Re: [EXT] Re: [virtio-comment] Re: [PATCH] virtio-net: Add equal-sized receive buffers feature
On Mon, Nov 25, 2019 at 11:31:09AM +0800, Jason Wang wrote: > > On 2019/11/24 äå11:30, Michael S. Tsirkin wrote: > > 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? > > > > 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. > > > One question here is that, the minimal buffer size should depends on various > factors. E.g the ring size. Consider a 256 entries ring, the minimal size > should be 64K/256=256 ... I guess you are right. We can make this driver programmable I guess? Basically pass min_buf_len to the device. > > > 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? > > > It looks to me the feature proposal here is something related to the > descriptor pre fetching. In the case of packed virtqueue. Having > avail/producer index may help more or less here? > > Thanks Isn't this addressed by VIRTIO_F_NOTIFICATION_DATA? At least that's one of the things it's supposed to do ... > > > > > > > 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. > > > > > 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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