linux-kernel - Re: [PATCH] bitops: add

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Message-ID: <4FAA7939.6040706@zytor.com>
Date:	Wed, 09 May 2012 07:03:37 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Michael S. Tsirkin" <mst@...hat.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 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

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