Re: [Qemu-devel] qemu and software suspend

[Oliver Gerlich]

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ace schrieb:
> Hi.
> 
> Has somebody tried to hibernate Linux while qemu is running?
> Here is my experience:
> 1. I was running qemu 0.7.2 fullscreen (under X with SDL),
> on Linux 2.6.14 host, with Win95 quest. All was fine,
> together with kqemu.
> 2. I changed to a virtual console and initiated hibernate
> (software suspend2 2.2-rc9). Everything latest stuff :)
> 3. That's when weird things started. The kernel throwed
> a "BUG", with the usual instruction and stack dump. The
> offending process was kswapd0.
> 4. But it successfully hibernated anyway. This was my first
> kernel complaint at hibernating ever.
> 5. After resuming the machine later, it seemed to work fine.
> I switched to qemu, but it was almost dead. Actually, qemu
> was fine, but the guest OS was almost dead. I could move
> application windows a bit, click on things to select them,
> but all this stopped after a while (seconds) and it didn't
> respond again. But qemu was going along. I could toggle
> fullscreen and it was taking 100% CPU (which is correct
> here). The management console worked fine.
> 6. On the host side, I noticed kswapd0 (kernel swap thread)
> was killed, but the OS still swapped fine.
> 7. I couldn't wake win95 so I had to quit qemu.
> 
> My questions:
> 1. May there be a problem with the kqemu module? Does
> it have proper suspend/resume routines? Or it doesn't need
> them (it is not a device driver)?

Have you already tried to hibernate without haveing kqemu loaded? That
could narrow down the problem.

> 2. Can the problem also be inside Win95? What happens
> in qemu when the time in the host system suddenly changes?
> Mine skipped 20 hours. Does the time in the quest continue
> without skips, thus the guest has different time than the
> host? Before win95 completelly stopped responding, it showed
> the old time from before the suspending on the taskbar.
> Or does qemu synchronise the time to the host? But I doubt
> that, mine was drifting behind even without suspending, the
> machine is quite slow.

I notice that when I stop emulation (while eg. Win2k is running in qemu)
and resume qemu some hours later, the guest clock has paused and is then
wrong by some hours. You could try that as well with Win95 and see what
happens (you can stop emulation by executing "stop" in the qemu monitor,
and resume operation by running "c" in the monitor).

> 
> I'd like to ask if suspending with qemu is officially
> declared "A bad thing to do", or it should work, but I have
> something wrong with my system. I run latest Slackware 10.2
> on a Pentium MMX.

There's been a posting on that topic some weeks ago, with a link to
http://people.brandeis.edu/~jcoiner/qemu_idedma/qemu_dma_patch.html#suspend
You could try hibernating with that patch applied.

Oliver
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