Re: [Qemu-devel] TSC in qem[-kvm] 1.1+ and in-kernel irqchip

[Michael Tokarev <mjt@tls.msk.ru>]

On 10.08.2012 11:33, Gleb Natapov wrote:
> On Thu, Aug 09, 2012 at 10:27:43PM +0400, Michael Tokarev wrote:
>> As a follow-up to the patch "tsc: use kvmclock for
>> calibration".
>>
>> There's another problem reported by several users.
>> The sympthom is that grub does not show boot menu,
>> it boots default entry right away without any pause.
>>
>> After quite some debugging it turned out to be
>> TSC issue.  Grub uses tsc for its timeout handling.
>> When setting timeout to some very large value
>> (10000), I can see the counter is ticking backwards
>> at a very high speed, ticking from 10000 to 0 in
>> about 5 seconds.
>>
>> Running kvm -cpu host,-tsc forces grub to use
>> rtc clocksource, and the problem goes away.
>>
> Can you try -no-kvm-pit-reinjection please.

It does not help.  With -no-kvm-pit-reinjection, the
time in grub is ticking about 1000 times faster still,
just like without.

>> The most interesting thing is that this is a
>> problem new for qemu-kvm 1.1 (and is still
>> present in current git), 1.0 version had no
>> such issue.  And it only happens when in-kernel
>> irqchip is enabled -- running with -no-kvm-irqchip
>> also fixes the grub problem, so that tsc starts
>> counting "correctly" for grub again.
>>
> 1.0 work on the same kernel 1.1 doesn't?

Yes.  This issue does not look like kernel-dependent -
it happens the same way on 3.0, 3.2 and 3.5 kernels.
Note that upstream qemu 1.1+ is also affected, when
used with -machine pc,kernel_irqchip=on.

/mjt