Message-ID: <4FAF8941.2010308@redhat.com>
Date: Sun, 13 May 2012 13:13:21 +0300
From: Avi Kivity <avi@redhat.com>
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To: Christoph Lameter <cl@linux.com>
CC: "Michael S. Tsirkin" <mst@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        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, Marcelo Tosatti <mtosatti@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCHv2] bitops: add _local bitops
References: <20120509162819.GA21328@redhat.com> <alpine.DEB.2.00.1205101401010.18664@router.home>
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On 05/10/2012 10:04 PM, Christoph Lameter wrote:
> On Wed, 9 May 2012, 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.
> >
> > Add _local bitops for this purpose: define them
> > as non-atomics for x86 and (for now) atomics for
> > everyone else.
>
> Have you tried to use the this_cpu_ops for that purpose? They create the
> per cpu atomic instructions that you want without a lock prefix and can
> also relocate the per cpu pointer to the correct processor via a
> segment register prefix.
>
> There are no bit operations provided right now but those can either be
> improvised using this_cpu_cmpxchg or added.

this_cpu_xchg() should be sufficient, since only bit zero has any
meaning in our use case (so xchg with zero is equivalent to
test_and_clear_bit).


-- 
error compiling committee.c: too many arguments to function

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