Re: [PATCH] Add VIRTIO_RING_F_LARGE_INDIRECT_DESC

[Stefan Hajnoczi <stefanha@redhat.com>]

On Fri, Nov 19, 2021 at 02:21:12PM +0100, Christian Schoenebeck wrote:
> This new feature flag allows indirect descriptor tables to
> exceed the queue size.
> 
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
> Stefan noted that there should also be a numeric configuration field
> reflecting a precise limit of indirect descriptors. The question is where
> should that go to exactly? Some devices currently handle this in their
> device specific configuration space. Wouldn't it make sense to handle that
> in the common configuration space instead?

It could be added as a read-only struct virtio_pci_common_cfg le16
queue_indirect_size field. The same needs to be done for the other
transports.

> ---
>  content.tex    | 21 +++++++++++++++++++++
>  split-ring.tex |  2 +-
>  2 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/content.tex b/content.tex
> index 5d112af..b42a26b 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 pending messages being emplaced in the vring,

The word "buffers" is typically used instead of "messages".

> +  as well as the actual bulk data size being transmitted.
> +
> +  With this feature enabled, the Queue Size only limits the maximum amount
> +  of pending messages in the vring, but does not oppose a limit to the actual
> +  bulk data size being transmitted. Decoupling these two configuration

I think we're still limited by Queue Size in the virtqueue descriptor
table so this statement only applies when indirect descriptors are used:

  "... build data size being transmitted when indirect descriptors are used."

> +  parameters this way not only allows much larger bulk data being transferred
> +  per message, but also avoids complicated synchronization mechanisms if
> +  device only supports a very small amount of pending/active messages.
> +  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..5284635 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_LARGE_INDIRECT_DESC has been negotiated.
>  
>  A driver MUST NOT set both VIRTQ_DESC_F_INDIRECT and VIRTQ_DESC_F_NEXT
>  in \field{flags}.
> -- 
> 2.20.1
>

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