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Re: [Qemu-ppc] [Qemu-devel] qemu-system-ppc video artifacts since "tcg:

From: Paolo Bonzini
Subject: Re: [Qemu-ppc] [Qemu-devel] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution"
Date: Thu, 16 Mar 2017 18:00:19 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 16/03/2017 16:34, Gerd Hoffmann wrote:
> Well, not *that* simple.  vga checks the dirty bitmap with scanline
> granularity, like that:
>   foreach (scanline) {
>      if (get_dirty(scanline))
>         update_scanline()
>   }
>   reset_dirty(framebuffer)
> I suspect simply transforming that to
>   foreach (scanline) {
>      if (test_and_clear_dirty(scanline))
>        update_scanline()
>   }
> 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 ...
> Looping twice over all scanlines, with the first loop just figuring
> which scanlines are modified, then clear dirty bits, then update in a
> second loop should work I think.  It'll duplicate a bunch of code
> though, because in reality the loop isn't just three lines because of
> doublescan, interlave and other funky stuff coming from CGA
> compatibility.
> Given that probably pretty much every display adapter is affected I'd
> tend to take Alex patch for 2.9, then sort the mess in the 2.10 devel
> cycle and revert the patch when done.

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.


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