Re: [virtio-comment] [PATCH v4] virtio-vsock: add max payload size config field

[Halil Pasic <pasic@linux.ibm.com>]

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.

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

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