Re: [virtio-comment] Re: [PATCH v2 0/2] Add VIRTIO_RING_F_LARGE_INDIRECT_DESC

[Cornelia Huck <cohuck@redhat.com>]

On Wed, Dec 08 2021, Stefan Hajnoczi <stefanha@redhat.com> wrote:

> On Wed, Dec 08, 2021 at 01:26:54PM +0100, Christian Schoenebeck wrote:

>> One more thought: what about making the new 'queue_indirect_size' config field 
>> 32 bit wide instead of 16 bit? That would easily mitigate the issue of the 
>> aggregated limit discussed here, and would in general be more future safe, 
>> i.e. considering that there might be either nested/multi-level indirect 
>> descriptor tables or chained tables in future? The cost would be low, right?
>
> The MMIO transport has 32-bit fields, so there doesn't seem to be a
> strict requirement to use 16 bits for descriptor counts.
>
> I think you're right that the cost is low. Usually it's the access
> itself that carries a cost (a VM exit or bus transaction) and 2 vs 4
> byte transfers don't really matter.

For ccw, I think we'd simply include the value in a control block, and
it does not really matter whether that control block is 2 bytes
longer. So, no objection from me.