Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution"

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo