Re: xen tsc problems?

[Keir Fraser <keir.fraser@eu.citrix.com>]

On 13/07/2010 18:48, "Keir Fraser" <keir.fraser@eu.citrix.com> wrote:

>> I started to wonder why the guest is seeing such a big tsc warp when xen
>> is seeing 0, so I added more tracing and eventually I found out that the
>> value of v->arch.hvm_vcpu.stime_offset is significantly different
>> between the two vcpus and the difference increases after the scaling.
>> Then I added timer_mode=1 to my vm config file and the problem went
>> away.
>> I think that delay_for_missed_ticks shouldn't cause tsc scew in
>> the guest.
> 
> Well, timer_mode=1 is the default and I doubt in all seriousness that the
> other modes get any use or testing.

To give you an idea how long it's probably been broken, my suspicion is that
the culprit is xen-unstable:17716, which is over two years old. That patch
changed HVM time handling to base it more on Xen system time. The fact that
hvm_set_guest_time() no longer directly affects guest TSC is probably the
problem here. I think delay_for_missed_ticks might depend on that. Anyway,
I'm not certain but I'd put money on it.

 -- Keir