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

[Paolo Bonzini]

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.

Paolo