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Re: [Qemu-devel] [PATCHv2 1/3] virtio: add missing mb() on notification
From: |
David Gibson |
Subject: |
Re: [Qemu-devel] [PATCHv2 1/3] virtio: add missing mb() on notification |
Date: |
Wed, 25 Apr 2012 16:38:50 +1000 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Tue, Apr 24, 2012 at 03:46:25PM +0200, Paolo Bonzini wrote:
> Il 23/04/2012 15:19, Michael S. Tsirkin ha scritto:
[snip]
> > #elif defined(_ARCH_PPC)
> >
> > /*
> > - * We use an eieio() for a wmb() on powerpc. This assumes we don't
> > + * We use an eieio() for wmb() and mb() on powerpc. This assumes we don't
> > * need to order cacheable and non-cacheable stores with respect to
> > * each other
> > */
> > #define smp_wmb() asm volatile("eieio" ::: "memory")
> > +#define smp_mb() asm volatile("eieio" ::: "memory")
>
> smp_mb() is hwsync under PPC, but I would just trust GCC.
I assume you mean 'lwsync', no such thing as 'hwsync', afaik. And I
assume you're talking about the kernel here.
So, the situation in qemu is different from in the kernel, because as
far as I know there's no case in qemu where we need to synchronize
cacheable stores with non-cacheable (I/O) stores. eieio won't do
that, but it will order cacheable stores w.r.t. other cacheable
stores. I think __sync_synchronize() will be a 'sync', the most
heavyweight memory barrier on ppc.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
[Qemu-devel] [PATCHv2 2/3] virtio: add missing mb() on enable notification, Michael S. Tsirkin, 2012/04/23
[Qemu-devel] [PATCHv2 3/3] virtio: order index/descriptor reads, Michael S. Tsirkin, 2012/04/23