[virtio-comment] Re: [PATCH 1/2] Add VIRTIO_RING_F_INDIRECT_SIZE

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Dec 14 2021, Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> This new feature flag allows to decouple the maximum amount of
> descriptors in indirect descriptor tables from the Queue Size.
>
> Fixes: https://github.com/oasis-tcs/virtio-spec/issues/122
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  content.tex    | 21 +++++++++++++++++++++
>  split-ring.tex |  2 +-
>  2 files changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/content.tex b/content.tex
> index 5d112af..0aa4842 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -6693,6 +6693,27 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
>    transport specific.
>    For more details about driver notifications over PCI see \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Available Buffer Notifications}.
>  
> +  \item[VIRTIO_RING_F_INDIRECT_SIZE(40)] This feature indicates that the
> +  maximum amount of descriptors in indirect descriptor tables is independent
> +  from the Queue Size.
> +
> +  If this feature bit is not negotiated, then a driver MUST NOT create a

"MUST NOT" does only belong into normative sections... maybe you can
reword the last sentence in this paragraph a bit and move the "MUST NOT"
part into a normative section?

> +  descriptor chain longer than the Queue Size, which then also applies to
> +  indirect descriptor tables as in \ref{sec:Basic Facilities of a Virtio
> +  Device / Virtqueues / The Virtqueue Descriptor Table / Indirect
> +  Descriptors}. Which means without this feature, the Queue Size limits
> +  both the maximum amount of slots in the vring, as well as the actual
> +  bulk data size being transmitted per vring slot.
> +
> +  With this feature enabled, the Queue Size only limits the maximum amount
> +  of slots in the vring, but does not limit the actual bulk data size
> +  being transmitted when indirect descriptors are used. Decoupling these
> +  two configuration parameters this way not only allows much larger bulk data
> +  being transferred per vring slot, but also avoids complicated synchronization
> +  mechanisms if the device only supports a very small amount of vring slots. Due
> +  to the 16-bit size of a descriptor's "next" field there is still an absolute
> +  limit of $2^{16}$ descriptors per indirect descriptor table.
> +
>  \end{description}
>  
>  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> diff --git a/split-ring.tex b/split-ring.tex
> index bfef62d..ae2aeb8 100644
> --- a/split-ring.tex
> +++ b/split-ring.tex
> @@ -269,7 +269,7 @@ \subsubsection{Indirect Descriptors}\label{sec:Basic Facilities of a Virtio Devi
>  one table per descriptor).
>  
>  A driver MUST NOT create a descriptor chain longer than the Queue Size of
> -the device.
> +the device unless VIRTIO_RING_F_INDIRECT_SIZE has been negotiated.

This might already be enough of a normative statement, though.

>  
>  A driver MUST NOT set both VIRTQ_DESC_F_INDIRECT and VIRTQ_DESC_F_NEXT
>  in \field{flags}.


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