Re: About preemption timer

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-12-17 10:32, Arthur Chunqi Li wrote:
> Hi Jan and Paolo,
> 
> I've tried to use preemption timer in KVM to trap vcpu regularly, but
> there's something unexpected. I run a VM with 4 vcpus and give them
> the same preemption timer value (e.g. 1000000) with all bits set
> (activate/save bits), then reset the value in preemption time-out
> handler.
> 
> Thus I expected these vcpus trap regularly in some special turns. But
> I found that when the VM is not busy, some vcpus are trapped much less
> frequently than others. In Intel SDM, I noticed that preemption timer
> is only related to TSC, and I think all the vcpus should trap in a
> similar frequency.
> 
> Could u help me explain this phenomenon?

Are you on a CPU that has non-broken preemption timer support? Anything
prior Haswell is known to tick with arbitrary frequencies.

BTW, we will have to re-implement preemption timer support with the help
of a regular host timer due to the breakage when halting L2 (see my test
case).

Jan

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