Re: [PATCH v2 1/2] Add VIRTIO_RING_F_LARGE_INDIRECT_DESC

[Stefan Hajnoczi <stefanha@redhat.com>]

On Mon, Nov 29, 2021 at 02:22:52PM +0100, Christian Schoenebeck wrote:
> This new feature flag allows indirect descriptor tables to
> exceed 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..34fa23e 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_LARGE_INDIRECT_DESC(40)] This feature indicates that the
> +  amount of descriptors in an indirect descriptor table is allowed to exceed
> +  the Queue Size.
> +
> +  If this feature bit is not negotiated, then a driver MUST NOT create a
> +  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 oppose a limit to the actual bulk data

s/oppose a limit to/impose a limit on/ but a more concise version is:

  ... does not limit the actual bulk data size ...

> +  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 device only supports a very small amount of vring slots. Due to

s/if device/if the device/

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