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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Tue, 28 Mar 2017 15:40:59 +0200
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Subject: Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since
 "tcg: drop global lock during TCG code execution"
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: jan.kiszka@siemens.com, Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>, qemu-devel <qemu-devel@nongnu.org>, cota@braap.org, "qemu-ppc@nongnu.org" <qemu-ppc@nongnu.org>, bobby.prani@gmail.com, fred.konrad@greensocs.com, Alex =?ISO-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, rth@twiddle.net

  Hi,

> > Well, not *that* simple.  vga checks the dirty bitmap with scanline
> > granularity, like that:
> >=20
> >   foreach (scanline) {
> >      if (get_dirty(scanline))
> >         update_scanline()
> >   }
> >   reset_dirty(framebuffer)
> >=20
> > I suspect simply transforming that to
> >=20
> >   foreach (scanline) {
> >      if (test_and_clear_dirty(scanline))
> >        update_scanline()
> >   }
> >=20
> > is not going to fly due to page tracking working with page granularity.
> > With two subsequent scanlines within one page the second scanline will
> > never be updated because updating first clears the dirty bit of the
> > page ...

> You're right; an alternative is to copy the dirty bitmap to a local one
> and clear the global one (the dirty bitmap for an 8 MB full HD frame
> buffer is just 256 bytes).  With the right API to abstract the job, it
> should be relatively easy to fix all adapters.

Started looking into this.  So I guess you are thinking about a variant
of memory_region_test_and_clear_dirty and/or
cpu_physical_memory_test_and_clear_dirty
which returns a bitmap instead of a bool?  Plus some helper function
which use the returned bitmap to figure whenever a specific scanline has
been touched or not?

cheers,
  Gerd