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

[Anthony Liguori <aliguori@us.ibm.com>]

Avi Kivity wrote:
> 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.

Yup.

> 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 don't know that I follow what you mean by "terminates a request".  I'm 
not sure I know what problem you're talking about solving.

Regards,

Anthony Liguori