[Qemu-ppc] Bug in qemu-system-ppc in Windows using the SDL2 GUI

[Howard Spoelstra]

Hi,

Running qemu-system-ppc.exe in windows with the SDL2 GUI results in a
hangup. I bisected to this commit:

8bb93c6f99a42c2e0943bc904b283cd622d302c5 is the first bad commit
commit 8bb93c6f99a42c2e0943bc904b283cd622d302c5
Author: Alex Bennée <address@hidden>
Date:   Wed Mar 15 14:48:25 2017 +0000

    ui/console: ensure graphic updates don't race with TCG vCPUs

    Commit 8d04fb55..

      tcg: drop global lock during TCG code execution

    ..broke the assumption that updates to the GUI couldn't happen at the
    same time as TCG vCPUs where running. As a result the TCG vCPU could
    still be updating a directly mapped frame-buffer while the display
    side was updating. This would cause artefacts to appear when the
    update code assumed that memory block hadn't changed.

    The simplest solution is to ensure the two things can't happen at the
    same time like the old BQL locking scheme. Here we use the solution
    introduced for MTTCG and schedule the update as async_safe_work when
    we know no vCPUs can be running.

    Reported-by: Mark Cave-Ayland <address@hidden>
    Signed-off-by: Alex Bennée <address@hidden>
    Message-id: address@hidden
    Cc: BALATON Zoltan <address@hidden>
    Cc: Gerd Hoffmann <address@hidden>
    Cc: Paolo Bonzini <address@hidden>
    Signed-off-by: Alex Bennée <address@hidden>

    [ kraxel: updated comment clarifying the display adapters are buggy
              and this is a temporary workaround ]

    Signed-off-by: Gerd Hoffmann <address@hidden>

Best,
Howard