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Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + co

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] Re: [PATCH] qemu/virtio: make wmb compiler barrier + comments
Date: Wed, 11 Nov 2009 20:08:08 +0200
User-agent: Mutt/1.5.19 (2009-01-05)

On Thu, Nov 12, 2009 at 01:18:11AM +0800, Scott Tsai wrote:
> On Wed, Nov 11, 2009 at 10:08 PM, Michael S. Tsirkin <address@hidden> wrote:
> > On Wed, Nov 11, 2009 at 01:45:35PM +0000, Paul Brook wrote:
> >> If you don't need real barriers, then why does the kvm code have them?
> >
> > We need real barriers but AFAIK kvm does not have them :(
> > IOW: virtio is currently broken with kvm, and my patch did
> > not fix this. The comment that I added says as much.
> How about just using GCC's __sync__synchronize atomic builtin (if
> detected as available by configure)?
> It's a full memory barrier instead of just a write barrier,  for x86,
> it generates the same code as the current Linux mb() implementation:
> "mfence" on x86_64
> "lock orl $0x0,(%esp)" on x86 unless -march is specified to a
> processor with "mfence".
> PPC could continue to use "eieio" while other architectures could just
> default to __sync_synchronize

Hmm, on x86 that's more expensive than it needs to be...
We can also ifdef platforms that we care about ...

> I do have a newbie question, when exactly would vrtio have to handle
> concurrent access from multiple threads?
> My current reading of the code suggests:
> 1. when CONFIG_IOTHREAD is true
> 2. when CONFIG_KVM is true and the guest machine has multiple CPUs

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