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Message-ID: <20120509154734.GB20867@redhat.com>
Date: Wed, 9 May 2012 18:47:35 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Rob Landley <rob@...dley.net>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
Arnd Bergmann <arnd@...db.de>,
Andrew Morton <akpm@...ux-foundation.org>,
David Howells <dhowells@...hat.com>,
Akinobu Mita <akinobu.mita@...il.com>,
Alexey Dobriyan <adobriyan@...il.com>,
Herbert Xu <herbert@...dor.hengli.com.au>,
Stephen Rothwell <sfr@...b.auug.org.au>,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-arch@...r.kernel.org, Gleb Natapov <gleb@...hat.com>,
Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
Avi Kivity <avi@...hat.com>,
Marcelo Tosatti <mtosatti@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] bitops: add _local bitops
On Wed, May 09, 2012 at 07:03:37AM -0700, H. Peter Anvin wrote:
> On 05/09/2012 06:45 AM, Michael S. Tsirkin wrote:
> > kvm needs to update some hypervisor variables atomically
> > in a sense that the operation can't be interrupted
> > in the middle. However the hypervisor always runs
> > on the same CPU so it does not need any memory
> > barrier or lock prefix.
> >
> > At Peter Anvin's suggestion, add _local bitops for this purpose:
> > define them as non-atomics for x86 and (for now) atomics
> > for everyone else.
> >
> > Uses are not restricted to virtualization: they
> > might be useful to communicate with an interrupt
> > handler if we know that it's running on the same CPU.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>
> I don't think you can use the x86 nonatomics as-is, because they don't
> contain optimization barriers.
>
> -hpa
By the way, clear_bit on x86 does not seem to contain
an optimization barrier - is my reading correct?
Lock prefix does not affect the compiler, right?
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