Re: Frame buffer corruptions with KVM >= 2.6.36

Avi Kivity
2010-10-14T12:13:45Z

On 10/14/2010 02:10 PM, Jan Kiszka wrote:
> 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,

I didn't see any mmio writes to the framebuffer, so I don't think
coalescing plays a part here.

> 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).
>

What about -no-kvm?

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