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Date: Mon, 1 Dec 2014 13:53:18 +0000
From: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20141201135318.GB6744@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [RFC 2/6] bitmap: add atomic test and clear
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Juan Quintela <quintela@redhat.com>, qemu-devel@nongnu.org, rth@redhat.com


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On Thu, Nov 27, 2014 at 05:43:56PM +0100, Paolo Bonzini wrote:
>=20
>=20
> On 27/11/2014 13:29, Stefan Hajnoczi wrote:
> > +bool bitmap_test_and_clear_atomic(unsigned long *map, long start, long=
 nr)
> > +{
> > +    unsigned long *p =3D map + BIT_WORD(start);
> > +    const long size =3D start + nr;
> > +    int bits_to_clear =3D BITS_PER_LONG - (start % BITS_PER_LONG);
> > +    unsigned long mask_to_clear =3D BITMAP_FIRST_WORD_MASK(start);
> > +    unsigned long dirty =3D 0;
> > +    unsigned long old_bits;
> > +
> > +    while (nr - bits_to_clear >=3D 0) {
> > +        old_bits =3D atomic_fetch_and(p, ~mask_to_clear);
> > +        dirty |=3D old_bits & mask_to_clear;
> > +        nr -=3D bits_to_clear;
> > +        bits_to_clear =3D BITS_PER_LONG;
> > +        mask_to_clear =3D ~0UL;
> > +        p++;
> > +    }
> > +    if (nr) {
> > +        mask_to_clear &=3D BITMAP_LAST_WORD_MASK(size);
> > +        old_bits =3D atomic_fetch_and(p, ~mask_to_clear);
> > +        dirty |=3D old_bits & mask_to_clear;
> > +    }
> > +
> > +    return dirty;
> > +}
>=20
> Same here; you can use atomic_xchg, which is faster because on x86
> atomic_fetch_and must do a compare-and-swap loop.

Will fix in v2.

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