[Qemu-devel] qemu and software suspend

[Qemu-devel] qemu and software suspend

[ace]

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)?
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'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.

Peter

Re: [Qemu-devel] qemu and software suspend

[Oliver Gerlich]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

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
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.1 (GNU/Linux)

iD8DBQFDcljyTFOM6DcNJ6cRApZWAJ9fjUC0v/NBRzT7n1Znk2zOayan/wCfRVrh
PYHxM7Hz/jkejQ3T4val2zE=
=tpza
-----END PGP SIGNATURE-----

Re: [Qemu-devel] qemu and software suspend

[ace]

Oliver Gerlich wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> 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.
Yes, I have now find the problem. Coincidentally, I just upgraded the
kernel before trying suspending with qemu and the kswapd0 bustage was a
bug in it. It is now officially fixed per 2.6.14.2. I am using a
nondefault io sheduler, that's why nobody else has seen this.
Who would have thought there can be bugs in the Linux kernel :)

But still after fixing this problem, all other symptoms with qemu stay
valid.

>>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 have tried this, stopping qemu for several minutes and I couldn't see
any problems. Yes, the quests time is then wrong, but the quest OS
worked fine.

>>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.

Thanks, I missed this posting on the mailinglist somehow (and it was
only 1 month old...). There was also another patch for this problem, and
that seems to work (see next email from me).

Peter