Re: [PATCH v2 1/4] Add VIRTIO_RING_F_INDIRECT_SIZE

[Stefan Hajnoczi <stefanha@redhat.com>]

On Mon, Feb 21, 2022 at 05:25:00PM +0100, Christian Schoenebeck wrote:
> @@ -6847,6 +6851,30 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
>    that the driver can reset a queue individually.
>    See \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}.
>  
> +  \item[VIRTIO_RING_F_INDIRECT_SIZE(41)] This feature indicates that the
> +  Queue Indirect Size, i.e. the maximum amount of descriptors in indirect
> +  descriptor tables, is independent from the Queue Size.
> +
> +  Without this feature, the Queue Size limits the length of the descriptor
> +  chain, including indirect descriptor tables as in \ref{sec:Basic Facilities of
> +  a Virtio Device / Virtqueues / The Virtqueue Descriptor Table / Indirect
> +  Descriptors}, i.e. both the maximum amount of slots in the vring and the
> +  actual bulk data size 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. However the
> +  actual maximum amount supported by either device or driver might be less,
> +  and therefore the bus specific Queue Indirect Size value MUST additionally

s/bus/transport/

Otherwise:
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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