[Qemu-devel] Re: [PATCH 0/4] Block DMA helpers (v2)

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
>>
>> This requires that the generic layer be able to tell where a request 
>> ends; don't know if that's the case now.
>
> Maybe create a sub QEMUSGList from another QEMUSGList.  The other 
> option is passing an offset and size to anything that takes a 
> QEMUSGList.  You could potentially get smart with how sub SG lists 
> were managed by making them just store an internal offset/size.  
> Although things start to get overly complex at some stage.

Lifetime issues will kill us.  I note that something resembling this 
sort of cleverness is holding up copyless networking -- different parts 
of an skb have different lifetimes.

But I thought of something simpler: have virtio call virtio-* to inquire 
whether a ring entry terminates a request.  This would also simplify the 
virtio device emulations somewhat.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.