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

["Blue Swirl" <blauwirbel@gmail.com>]

On 12/8/07, Paul Brook <paul@codesourcery.com> wrote:
> 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.

If we start adding locks for IO, could we use the same locking model
more widely or make it generic so that it would support a SMP host as
well?