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

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

On 14/03/17 15:41, Alex Bennée wrote:

> BALATON Zoltan <balaton@eik.bme.hu> writes:
> 
>> On Tue, 14 Mar 2017, Mark Cave-Ayland wrote:
>>> On 14/03/17 10:00, Alex Bennée wrote:
>>>> Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> writes:
>>>>
>>>>> I've recently noticed some video artifacts appearing in the form of
>>>>> horizontal lines whilst testing OpenBIOS boot on some qemu-system-ppc
>>>>> images (see https://www.ilande.co.uk/tmp/qemu/macos9-stripe.png for an
>>>>> example) which I've finally bisected down to this commit.
>>>>
>>>> Interesting. Is the video hardware in this case a simple framebuffer or
>>>> is there some sort of GPU involved?
>>>
>>> For the mac99 machine it's just the normal QEMU PCI std-vga card, so
>>> nothing special. Having run tests across SPARC which uses its own
>>> framebuffer, there have been no obvious artifacts that I've spotted
>>> there to date.
>>>
>>>>> The horizontal lines tend to appear at different positions with
>>>>> different lengths, although it seems to be more common that a complete
>>>>> scanline is affected. Reproducing the issue is fairly easy with a MacOS
>>>>> 9.2.1 ISO and the command line below:
>>>>>
>>>>> ./qemu-system-ppc -cdrom MacOS921.iso -boot d -m 512 -M mac99
>>>>>
>>>>> Could it be that this patch is still missing a lock somewhere which
>>>>> causes these artifacts to appear?
>>>>
>>>> It depends. If the hardware is accessed via MMIO then the BQL is taken
>>>> automatically (unless the area is explicitly marked as doing its own
>>>> locking see: mr->global_locking).
>>>>
>>>> Is the artefact temporary or permanent? Could it be a change in
>>>> synchronisation between the emulator updating the framebuffer and
>>>> whatever backend updating its display?
>>>
>>> AFAICT they are temporary in that they appear randomly on the display,
>>> but the next time that particular area of the display is updated by the
>>> guest then they tend to either change/disappear.
>>
>> I've also noticed artifacts when testing the SM501 changes with
>> MorphOS on recent QEMU so it may not be related to just the std vga.
>> It's shown as bands (larger number of adjacent scanlines) not updating
>> when the client first wrote to the framebuffer but then they
>> disappeared during the next refresh. I'm guessing it may be related to
>> dirty checking of the framebuffer memory which is used to decide when
>> a scan line needs update?
> 
> Interesting. I guess if the corrupted scan lines are in blocks of
> PAGE_SIZE that might make sense.
> 
> Commit (b0706b716769494f321a0d2bfd9fa9893992f995) made
> tlb_reset_dirty_range update the SoftMMU addr_write entry atomically.
> Now I don't see how that could race in single threaded TCG mode but it
> could explain things.
> 
> I notice that tlb_set_dirty/tlb_set_dirty1 should maybe do the same. The
> currently assume they are only called from the CPU's context. If you
> enable #define DEBUG_TLB in cputlb.c does the assert fire?

Not in a quick local test here. However this does ring a bell - one of
Ben's PPC optimisations was to batch TLB flushes so they only occur at
certain times - see commit cd0c6f473532bfaf20a095bc90a18e45162981b5 for
more detail.

Could it be that these underlying assumptions have now changed?


ATB,

Mark.