Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device

[Paul Brook <paul@codesourcery.com>]

On Saturday 08 December 2007, Jamie Lokier wrote:
> Paul Brook wrote:
> > > virtio makes things a bit trickier though.  There's a shared ring queue
> > > between the host and guest.  The ring queue is lock-less and depends on
> > > the ability to atomically increment ring queue indices to be SMP safe.
> > > Using a copy-API wouldn't be a problem for QEMU since the host and
> > > guest are always running in lock-step.  A copy API is actually needed
> > > to deal with differing host/guest alignment and endianness.
> >
> > That seems a rather poor design choice, as many architectures don't have
> > an atomic increment instruction. Oh well.
>
> Most have compare-and-swap or load-locked/store-conditional
> instructions, though, which can be used to implement atomic increment.

Yes, but your "hardware" implementation has to make sure it interacts with 
those properly. It's certainly possible to implement lockless lists without 
requiring atomic increment. Most high-end hardware manages it and that 
doesn't even have coherent DMA.

Paul