[Qemu-devel] linux verify_pmtmr_rate() issue

[Qemu-devel] linux verify_pmtmr_rate() issue

[Matthew Kent]

Issue here that's beyond my skill set to resolve:

I've been starting multiple linux 2.6.23-rc3 x86 guests up in parallel
with qemu/kvm and noticed pm-timer is being disabled in some of them
with

PM-Timer running at invalid rate: 126% of normal - aborting.

in dmesg when I start about 6 at a time. Unfortunately without the timer
a tickless kernel in my guests is disabled. 

I also replicated the issue by starting a single vm when the host system
was busy enough.

After some amateurish debugging added to verify_pmtmr_rate() in the
kernel acpi_pm driver and get_pmtmr() in qemu acpi I can indeed see it
returning just slowly enough to throw off the sanity check. 

[   10.264772] DEBUG: PM-Timer running value1: 2925874 value2: 3058371
expected_rate: 107385 delta: 132497 count: 2269
[   10.270766] PM-Timer running at invalid rate: 123% of normal -
aborting.

For now I've just disabled verify_pmtmr_rate() in the kernel for my
guests and they seem to be keeping time just fine. 

Not sure if a patch for the linux kernel making the sanity check
optional with a kernel parameter would make sense or there's something
else that can be done at the qemu level.

Thanks.
-- 
Matthew Kent <mkent@magoazul.com>
http://magoazul.com

[Qemu-devel] Re: [kvm-devel] linux verify_pmtmr_rate() issue

[Avi Kivity]

Matthew Kent wrote:
> Issue here that's beyond my skill set to resolve:
>
> I've been starting multiple linux 2.6.23-rc3 x86 guests up in parallel
> with qemu/kvm and noticed pm-timer is being disabled in some of them
> with
>
> PM-Timer running at invalid rate: 126% of normal - aborting.
>
> in dmesg when I start about 6 at a time. Unfortunately without the timer
> a tickless kernel in my guests is disabled. 
>
> I also replicated the issue by starting a single vm when the host system
> was busy enough.
>
> After some amateurish debugging added to verify_pmtmr_rate() in the
> kernel acpi_pm driver and get_pmtmr() in qemu acpi I can indeed see it
> returning just slowly enough to throw off the sanity check. 
>
> [   10.264772] DEBUG: PM-Timer running value1: 2925874 value2: 3058371
> expected_rate: 107385 delta: 132497 count: 2269
> [   10.270766] PM-Timer running at invalid rate: 123% of normal -
> aborting.
>
> For now I've just disabled verify_pmtmr_rate() in the kernel for my
> guests and they seem to be keeping time just fine. 
>
> Not sure if a patch for the linux kernel making the sanity check
> optional with a kernel parameter would make sense or there's something
> else that can be done at the qemu level.
>   

You can try implementing qemu's cpu_get_real_ticks() using
gettimeofday() instead of using the time stamp counter (which can go
back or jump forward if the time stamp counter is not synced across
cpus).  Not sure if that's the problem though.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.