Re: [PATCH v2] Shared memory device with interrupt support

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
> I'd strongly recommend working these patches on qemu-devel and lkml.  
> I suspect Avi may disagree with me, but in order for this to be 
> eventually merged in either place, you're going to have additional 
> requirements put on you.

I don't disagree with the fact that there will be additional 
requirements, but I might disagree with some of those additional 
requirements themselves.  In particular I think your proposal was 
unimplementable; I would like to see how how you can address my concerns.

I don't think bulk memory sharing and the current transactional virtio 
mechanisms are a good fit for each other; but if we were to add a 
BAR-like capability to virtio that would address the compatibility 
requirement (though it might be difficult to implement on s390 with its 
requirement on contiguous host virtual address space).

A model which does fit the current virtio capabilities is that of a DMA 
engine - guest A specifies an sglist to copy; guest B specifies an 
sglist to receive the copy; the host does the copy, using a real DMA 
engine if available.  Note A == B is a possibility, and is a way to 
expose a DMA engine to a single guest for its own use in moving memory 
around.

I think both models could prove useful.

> If it goes in via qemu-kvm.git, there's a possibility that you'll be 
> forced into an ABI break down the road (consider the old hypercall and 
> balloon drivers).

I agree this is best merged upstream first.

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