Re: [kvm-devel] [RFC PATCH 0/4] Inter-guest virtio I/O example with lguest

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>> Avi Kivity wrote:
>>
>> Each guest's host userspace mmaps the other guest's address space.  
>> The userspace then does a copy on both the tx and rx paths.
>>
>
> Well, that's better security-wise (I'd still prefer to avoid it, so we 
> can run each guest under a separate uid), but then we lose performance 
> wise.

What performance win?  I'm not sure the copies can be eliminated in the 
case of interguest IO.

Fast interguest IO means mmap()'ing the other guest's address space 
read-only.  If you had a pv dma registration api you could conceivably 
only allow the active dma entries to be mapped but my fear would be that 
the zap'ing on unregister would hurt performance.

>> Conceivably, this could be done as a read-only mapping so that each 
>> guest userspace copies only the rx packets.  That's about as secure 
>> as you're going to get with this approach I think.
>>
>
> Maybe we can terminate the virtio queue in the host kernel as a pipe, 
> and splice pipes together.
>
> That gives us guest-guest and guest-process communications, and if you 
> use aio the kernel can use a dma engine for the copy.

Ah, so you're looking to use a DMA engine for accelerated copy.  Perhaps 
the answer is to expose the DMA engine via a userspace API?

Regards,

Anthony Liguori