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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: Tue, 28 Mar 2017 16:13:59 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 28/03/2017 15:40, Gerd Hoffmann wrote:
>   Hi,
>>> 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 ...
>> 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?

Yes, exactly.


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