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

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
>> 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".  

A request is composed of multiple virtio ring elements.  The last ring 
entry for a given request terminates it.

> I'm not sure I know what problem you're talking about solving.

The Linux guest interface is at the request level, accepting 
scatter/gather lists as parameters.  The qemu host interface is at the 
ring element level, leaving the task of gathering ring elements to make 
a request (and gathering segments to make a scatter/gather list) to users.

I think we should make the qemu virtio host interface talk at the 
request level, rather than the ring entry level.  If we do that, we'll 
get a cleaner, easier to use interface.

But I think I know what you're getting at: virtio sglists contain a 
descriptor followed by data.  We could handle that by adding

  qemu_sglist_sublist(QEMUSGList *sg, QEMUSGList *sublist, 
target_phys_addr_t offset, target_phys_addr_t len);

it would involve extra copying and allocation, but we could live with 
it, I think.

-- 
error compiling committee.c: too many arguments to function