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Subject: Re: [virtio-comment] [PATCH v4] virtio-vsock: add max payload size config field
On 6/27/22 16:15, Michael S. Tsirkin wrote:
CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe. On Mon, Jun 27, 2022 at 02:59:07PM +0200, Halil Pasic wrote:On Fri, 24 Jun 2022 11:23:31 +0300 Laura Loghin <lauralg@amazon.com> wrote:On 6/23/22 19:22, Michael S. Tsirkin wrote:CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe. On Thu, Jun 23, 2022 at 06:36:50PM +0300, Laura Loghin wrote:On 6/16/22 19:40, Michael S. Tsirkin wrote: CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe. On Tue, Jun 14, 2022 at 01:57:34PM +0200, Halil Pasic wrote: On Mon, 13 Jun 2022 13:40:38 +0300 Laura Loghin<lauralg@amazon.com> wrote: @@ -57,6 +62,25 @@ \subsection{Device configuration layout}\label{sec:Device Types / Socket Device \hline \end{tabular} +The following driver-read-only field, \field{data_max_size} only exists if +VIRTIO_VSOCK_F_SIZE_MAX is set. This field specifies the maximum packet payload +size for the driver to use. + +\devicenormative{\subsubsection}{Device configuration layout}{Device Types / Socket Device / Device configuration layout} + +The device MUST NOT change the value exposed through \field{data_max_size}. + +\drivernormative{\subsubsection}{Device configuration layout}{Device Types / Socket Device / Device configuration layout} + +A driver SHOULD negotiate VIRTIO_VSOCK_F_SIZE_MAX if the device offers it. + +If the driver negotiates VIRTIO_VSOCK_F_SIZE_MAX, the receive buffers it +supplies for a packet MUST have a total size that doesn't exceed the size +\field{data_max_size} (plus header length). + +If the driver negotiates VIRTIO_VSOCK_F_SIZE_MAX, it MUST NOT transmit packets +of size exceeding the value of \field{data_max_size} (plus header length). + Hi and sorry for being late to the party! I believe I do understand why do we want to put a restriction on the size of the transmitted packets, but I would appreciate if you could explain to me why do we want to limit the receive buffer size. Also I find the wording regarding a little bit ambiguous because in a networking context it also makes sense to talk about the size of the receive buffer. I guess hear we are talking about a single virtio buffer (a descriptor chain described potentially non-continuous (or compact in the mathematical sense of the word) which is composed from as many continuous chunks of memory as many descriptors are contained within the descriptor chain). If we are indeed talking about a single virtio buffer, I don't understand the plural. If not, I'm not sure what are we talking about. I think I agree here, I don't understand the mix of "buffers" and "a packet" either. The way I was understanding that while reading the spec is that a buffer is corresponding to one descriptor,what gave this impression? buffers can use any number of descriptors.There are sections in the spec that make you think of a buffer as corresponding to a descriptor chain, and others that don't, for example: 2.6.5 The Virtqueue Descriptor Table: Each descriptor describes a buffer which is read-only for the device (âdevice-readableâ) or write-only for the device (âdevice-writableâ), but a chain of descriptors can contain both device-readable and device-writable buffers. I remember while reading the spec I was always confused about the buffer - descriptor (chain) relation, but in the end concluded that the buffer is the memory region to which a single descriptor is pointing to (probably also because when you think of a buffer you would at first assume it is contiguous in memory).well we have wording like Device reports the number of bytes it has written to memory for each buffer it uses. This is referred to as ``used length''.
That's true, there are also sections that make you think of a buffer belonging to the entire descriptor chain. Hopefully once we update the spec, the confusion will no longer be possible.
I think in the distant past I have pointed out that the usage of the word buffer in the virtio spec is somewhat confusing. But I never took the time to make a patch which attempts to set it strait. IMHO eve if we were to use "buffer" consistently in a sense that, it is a portion of the RAM which may or may not be continuous in the address space used by virtio (guest physical or DMA depending on ...) where chained descriptors describe continuous pieces of the same, we would still have the problem, that in the normal CS world AFAIK a buffer is usually a continuous piece of memory used for a certain purpose. https://en.wikipedia.org/wiki/Data_buffer Well actually our buffers are continuous in a sense that each descriptor chain defines and has an address space of its own. In a sense that we can talk about the N-th byte of the L long buffer (N < L), but that the difference between addresses of the 0-th and the N-th byte of an virtio buffer described by a descriptor chain in DMA/guest physical address space isn't guaranteed to be N. One solution would be to define the terms: virtqueue buffer: a portion of a RAM that is composed by one or more primitive buffers and a descriptor chain such that at least one descriptor in the chain is pointed to each primitive buffer primitive buffer: a byte array in RAM which is continuous in the address space used by the spec and use this terms consistently. That is never use buffer without a qualifier.I would be inclined to avoid saying primitive buffer. saying virtqueue buffer is ok if a bit verbose. Let's start by locating places in spec which say buffer when they mean descriptor though.BTW I don't remember if the our primitive buffers are allowed to cross page boundaries.Spec doesn't say they shouldn't.This doesn't seem to be the right understanding, so I will update the patch.so a packet will correspond to multiple buffers (like for example in Linux one buffer for the packet header and one buffer for the payload). I wanted to limit the memory allocated by the driver for RX buffers and TX buffers, so that's why I used 'buffers' for RX. Does it make sense or did I misunderstand what was causing the confusion here? Thanks, LauraAs above, a buffer can consist of many descriptors. See e.g. Descriptor Chaining. I think there are places in spec when say "descriptor" and we should fix them to say one or more descriptors.It would be great if we could fix the spec, so that the relation between a descriptor and a buffer becomes more clear and doesn't leave room for interpretations.+1 Regards, HalilLauraI voted "no" on the ballot, though if others feel we should apply as is and fix up later, that is not too bad. Also, do we have some sort of packets may not cross virtio buffer boundaries, or even a single packet per single viritio buffer rule for vsock. I did a quick search and could not find any. Did I overlook something? Should we spell this out? @Michael, Conny: What do you think? Regards, Halil Amazon Development Center (Romania) S.R.L. registered office: 27A Sf. Lazar Street, UBC5, floor 2, Iasi, Iasi County, 700045, Romania. Registered in Romania. Registration number J22/2621/2005.Amazon Development Center (Romania) S.R.L. registered office: 27A Sf. Lazar Street, UBC5, floor 2, Iasi, Iasi County, 700045, Romania. Registered in Romania. Registration number J22/2621/2005.
Amazon Development Center (Romania) S.R.L. registered office: 27A Sf. Lazar Street, UBC5, floor 2, Iasi, Iasi County, 700045, Romania. Registered in Romania. Registration number J22/2621/2005.
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