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From: "Michael S. Tsirkin" <mst@redhat.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Rob Landley <rob@landley.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	x86@kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Howells <dhowells@redhat.com>,
	Akinobu Mita <akinobu.mita@gmail.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Herbert Xu <herbert@gondor.hengli.com.au>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, Gleb Natapov <gleb@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, Avi Kivity <avi@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] bitops: add _local bitops
Date: Wed, 9 May 2012 18:47:35 +0300	[thread overview]
Message-ID: <20120509154734.GB20867@redhat.com> (raw)
In-Reply-To: <4FAA7939.6040706@zytor.com>

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@redhat.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?

  parent reply	other threads:[~2012-05-09 16:21 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-05-09 13:45 [PATCH] bitops: add _local bitops Michael S. Tsirkin
2012-05-09 14:03 ` H. Peter Anvin
2012-05-09 15:06   ` Michael S. Tsirkin
2012-05-09 15:43   ` Michael S. Tsirkin
2012-05-09 15:44     ` H. Peter Anvin
2012-05-09 15:47   ` Michael S. Tsirkin [this message]
2012-05-09 16:24     ` H. Peter Anvin
2012-05-09 16:36       ` Michael S. Tsirkin
2012-05-09 16:45         ` H. Peter Anvin
2012-05-09 16:55           ` Michael S. Tsirkin
2012-05-09 14:06 ` Arnd Bergmann
2012-05-09 14:17 ` Avi Kivity
2012-05-09 19:19 ` Andrew Morton
2012-05-09 19:23   ` H. Peter Anvin
2012-05-09 20:07   ` Michael S. Tsirkin
2012-05-09 20:10     ` H. Peter Anvin
2012-05-09 20:12       ` Michael S. Tsirkin
2012-05-10  9:26         ` Avi Kivity
2012-05-10 23:02       ` Benjamin Herrenschmidt
2012-05-10 17:38 ` Rob Landley

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