[virtio-comment] Re: [PATCH v1 1/2] virtio-net: Fix receive buffer size calculation text

[Cornelia Huck <cohuck@redhat.com>]

On Fri, Jan 05 2024, Parav Pandit <parav@nvidia.com> wrote:

> Receive buffer size calculation is based on the following
> negotiated features.
>
> The text has wrong calculation for IPv6 and also it has missed
> VIRTIO_NET_F_HASH_REPORT.
>
> The problem of igorance of VIRTIO_NET_F_HASH_REPORT is reported
> in [1], however fix for ipv6 payload length must also be
> considered.
>
> Since for the both the fixes touching same requirements, a
> new issue is created as [2].
>
> This patch brings following fixes.
>
> 1. Fix annotating struct virtio_net_hdr as field
> 2. Fix receive buffer calculation for guest GSO cases to consider
>    ipv6 payload length
> 3. small grammar corrections for article
> 4. reword the requirement to consider the virtio_ndr_hdr which is
>    depends on the negotiated feature, hence first clarify the
>    struct virtio_net_hdr size
>
> [1] https://github.com/oasis-tcs/virtio-spec/issues/170
> [2] https://github.com/oasis-tcs/virtio-spec/issues/183
>
> Fixes: https://github.com/oasis-tcs/virtio-spec/issues/170
> Fixes: https://github.com/oasis-tcs/virtio-spec/issues/183
> Signed-off-by: Parav Pandit <parav@nvidia.com>
> ---
>  device-types/net/description.tex | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index aff5e08..a1f40f7 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -657,24 +657,35 @@ \subsubsection{Setting Up Receive Buffers}\label{sec:Device Types / Network Devi
>  If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6,
>  VIRTIO_NET_F_GUEST_UFO, VIRTIO_NET_F_GUEST_USO4 or VIRTIO_NET_F_GUEST_USO6
>  features are used, the maximum incoming packet
> -will be to 65550 bytes long (the maximum size of a
> -TCP or UDP packet, plus the 14 byte ethernet header), otherwise
> -1514 bytes.  The 12-byte struct virtio_net_hdr is prepended to this,
> -making for 65562 or 1526 bytes.
> +will of size 65589 bytes long (14 bytes of Ethernet header, plus 40 bytes of

Either "will be 65589 bytes long" or "will be of 65589 bytes size" (I'd
prefer the former.)

> +the IPv6 header, plus 65535 bytes of maximum IPv6 payload including any
> +extension header) otherwise 1514 bytes.

s/ otherwise/, otherwise/

> +When VIRTIO_NET_F_HASH_REPORT is not negotiated, the required receive buffer
> +size is either 65601 or 1526 bytes.
> +When VIRTIO_NET_F_HASH_REPORT is negotiated, the required receive buffer
> +size is either 65609 or 1534 bytes.
>  
>  \drivernormative{\paragraph}{Setting Up Receive Buffers}{Device Types / Network Device / Device Operation / Setting Up Receive Buffers}
>  
> +If VIRTIO_NET_F_HASH_REPORT is not negotiated, the size of the field
> +\field{struct virtio_net_hdr} is 12 bytes.
> +
> +If VIRTIO_NET_F_HASH_REPORT is negotiated, the size of the field
> +\field{struct virtio_net_hdr} is 20 bytes.

This reads a bit odd because these two are not normative statements. I'd
rather include this information into the actual statements.

> +
>  \begin{itemize}
>  \item If VIRTIO_NET_F_MRG_RXBUF is not negotiated:
>    \begin{itemize}
>      \item If VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6, VIRTIO_NET_F_GUEST_UFO,
>  	VIRTIO_NET_F_GUEST_USO4 or VIRTIO_NET_F_GUEST_USO6 are negotiated, the driver SHOULD populate
> -      the receive queue(s) with buffers of at least 65562 bytes.
> +      the receive queue(s) with buffers of at least 65589 bytes plus the size
> +      of the \field{struct virtio_net_hdr}.

"with buffers of at least 65609 bytes if VIRTIO_NET_F_HASH_REPORT is
negotiated, and of at least 65601 bytes if not."

>      \item Otherwise, the driver SHOULD populate the receive queue(s)
> -      with buffers of at least 1526 bytes.
> +      with buffers of at least 1514 bytes plus the size of the
> +      \field{struct virtio_net_hdr}.

"with buffers of at least 1534 bytes if VIRTIO_NET_F_HASH_REPORT is
negotiated, and of at least 1526 bytes if not."

>    \end{itemize}
>  \item If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer MUST be at
> -least the size of the struct virtio_net_hdr.
> +least the size of the \field{struct virtio_net_hdr}.

If you reformat it like that, I'd drop the "the", and also state the
lenght explictly:

"each buffer MUST be at least the size of \field{struct virtio_net_hdr},
i.e. 20 bytes if VIRTIO_NET_F_HASH_REPORT is negotiated, and 12 bytes if
not."


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