On Thu, 19 Jan 2023, Akihiko Odaki wrote:
On 2023/01/15 3:11, BALATON Zoltan wrote:
On Sat, 14 Jan 2023, Akihiko Odaki wrote:
On 2023/01/13 22:43, BALATON Zoltan wrote:
On Thu, 5 Jan 2023, BALATON Zoltan wrote:
Hello,
I got reports from several users trying to run AmigaOS4 on sam460ex on
Apple silicon Macs that they get missing graphics that I can't reproduce
on x86_64. With help from the users who get the problem we've narrowed
it down to the following:
It looks like that data written to the sm501's ram in
qemu/hw/display/sm501.c::sm501_2d_operation() is then not seen from
sm501_update_display() in the same file. The sm501_2d_operation()
function is called when the guest accesses the emulated card so it may
run in a different thread than sm501_update_display() which is called by
the ui backend but I'm not sure how QEMU calls these. Is device code
running in iothread and display update in main thread? The problem is
also independent of the display backend and was reproduced with both
-display cocoa and -display sdl.
We have confirmed it's not the pixman routines that sm501_2d_operation()
uses as the same issue is seen also with QEMU 4.x where pixman wasn't
used and with all versions up to 7.2 so it's also not some bisectable
change in QEMU. It also happens with --enable-debug so it doesn't seem
to be related to optimisation either and I don't get it on x86_64 but
even x86_64 QEMU builds run on Apple M1 with Rosetta 2 show the problem.
It also only seems to affect graphics written from sm501_2d_operation()
which AmigaOS4 uses extensively but other OSes don't and just render
graphics with the vcpu which work without problem also on the M1 Macs
that show this problem with AmigaOS4. Theoretically this could be some
missing syncronisation which is something ARM and PPC may need while x86
doesn't but I don't know if this is really the reason and if so where
and how to fix it). Any idea what may cause this and what could be a fix
to try?
Any idea anyone? At least some explanation if the above is plausible or
if there's an option to disable the iothread and run everyting in a
single thread to verify the theory could help. I've got reports from at
least 3 people getting this problem but I can't do much to fix it without
some help.
(Info on how to run it is here:
http://zero.eik.bme.hu/~balaton/qemu/amiga/#amigaos
but AmigaOS4 is not freely distributable so it's a bit hard to
reproduce. Some Linux X servers that support sm501/sm502 may also use
the card's 2d engine but I don't know about any live CDs that readily
run on sam460ex.)
Thank you,
BALATON Zoltan
Sorry, I missed the email.
Indeed the ui backend should call sm501_update_display() in the main
thread, which should be different from the thread calling
sm501_2d_operation(). However, if I understand it correctly, both of the
functions should be called with iothread lock held so there should be no
race condition in theory.
But there is an exception: memory_region_snapshot_and_clear_dirty()
releases iothread lock, and that broke raspi3b display device:
https://lore.kernel.org/qemu-devel/CAFEAcA9odnPo2LPip295Uztri7JfoVnQbkJ=wn+k8dqneb_...@mail.gmail.com/T/
It is unexpected that gfx_update() callback releases iothread lock so it
may break things in peculiar ways.
Peter, is there any change in the situation regarding the race introduced
by memory_region_snapshot_and_clear_dirty()?
For now, to workaround the issue, I think you can create another mutex and
make the entire sm501_2d_engine_write() and sm501_update_display()
critical sections.
Interesting thread but not sure it's the same problem so this workaround
may not be enough to fix my issue. Here's a video posted by one of the
people who reported it showing the problem on M1 Mac:
https://www.youtube.com/watch?v=FDqoNbp6PQs
and here's how it looks like on other machines:
https://www.youtube.com/watch?v=ML7-F4HNFKQ
There are also videos showing it running on RPi 4 and G5 Mac without this
issue so it seems to only happen on Apple Silicon M1 Macs. What's strange
is that graphics elements are not just delayed which I think should happen
with missing thread synchronisation where the update callback would miss
some pixels rendered during it's running but subsequent update callbacks
would eventually draw those, woudn't they? Also setting full_update to 1 in
sm501_update_display() callback to disable dirty tracking does not fix the
problem. So it looks like as if sm501_2d_operation() running on one CPU
core only writes data to the local cache of that core which
sm501_update_display() running on other core can't see, so maybe some cache
synchronisation is needed in memory_region_set_dirty() or if that's already
there maybe I should call it for all changes not only those in the visible
display area? I'm still not sure I understand the problem and don't know
what could be a fix for it so anything to test to identify the issue better
might also bring us closer to a solution.
Regards,
BALATON Zoltan
If you set full_update to 1, you may also comment out
memory_region_snapshot_and_clear_dirty() and
memory_region_snapshot_get_dirty() to avoid the iothread mutex being
unlocked. The iothread mutex should ensure cache coherency as well.
But as you say, it's weird that the rendered result is not just delayed but
missed. That may imply other possibilities (e.g., the results are overwritten
by someone else). If the problem persists after commenting out
memory_region_snapshot_and_clear_dirty() and
memory_region_snapshot_get_dirty(), I think you can assume the inter-thread
coherency between sm501_2d_operation() and sm501_update_display() is not
causing the problem.
I've asked people who reported and can reproduce it to test this but it
did not change anything so confirmed it's not that race condition but
looks more like some cache inconsistency maybe. Any other ideas?
Regards,
BALATON Zoltan
