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Date: Sat, 8 Dec 2007 14:09:06 +0000
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device
Message-ID: <20071208140906.GA22712@shareable.org>
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To: qemu-devel@nongnu.org
Cc: Anthony Liguori <aliguori@us.ibm.com>, Rusty Russell <rusty@rustcorp.com.au>

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.

-- Jamie