Re: Frame buffer corruptions with KVM >= 2.6.36

[Jan Kiszka <jan.kiszka@siemens.com>]

Am 14.10.2010 09:27, Jan Kiszka wrote:
> Hi,
> 
> I'm seeing quite frequent corruptions of the VESA frame buffer with
> Linux guests (vga=0x317) that are starting with KVM kernel modules of
> upcoming 2.6.36 (I'm currently running -rc7). Effects disappears when
> downgrading to kvm-kmod-2.6.35.6. Will see if I can bisect later, but
> maybe someone already has an idea or wants to reproduce (just run
> something like "find /" on one text console and witch to another one -
> text fragments will remain on the screen on every few switches).

Commit d25f31f488e5f7597c17a3ac7d82074de8138e3b in kvm.git ("KVM: x86:
avoid unnecessary bitmap allocation when memslot is clean") is at least
magnifying the issue. With this patch applied, I can easily trigger
display corruptions when switching between VGA consoles while one of
them is undergoing heavy updates.

However, I once saw a much smaller inconsistency during my tests even
with a previous revision. Maybe there is a fundamental issue in when and
how the coalesced backlog is replayed, and this commit just makes the
corruptions more likely. This may even be a QEMU issue in the cirrus/vga
model (both qemu-kvm and upstream show the effect).

Jan

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