Re: [PATCH v5] KVM: nVMX: Fully support of nested VMX preemption timer

[Jan Kiszka <jan.kiszka@web.de>]

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On 2013-09-26 17:04, Paolo Bonzini wrote:
> Il 16/09/2013 10:11, Arthur Chunqi Li ha scritto:
>> This patch contains the following two changes:
>> 1. Fix the bug in nested preemption timer support. If vmexit L2->L0
>> with some reasons not emulated by L1, preemption timer value should
>> be save in such exits.
>> 2. Add support of "Save VMX-preemption timer value" VM-Exit controls
>> to nVMX.
>>
>> With this patch, nested VMX preemption timer features are fully
>> supported.
>>
>> Signed-off-by: Arthur Chunqi Li <yzt356@gmail.com>
>> ---
>> ChangeLog to v4:
>> 	Format changes and remove a flag in nested_vmx.
>>  arch/x86/include/uapi/asm/msr-index.h |    1 +
>>  arch/x86/kvm/vmx.c                    |   44 +++++++++++++++++++++++++++++++--
>>  2 files changed, 43 insertions(+), 2 deletions(-)
> 
> Hi all,
> 
> the test fails for me if the preemption timer value is set to a value
> that is above ~2000 (which means ~65000 TSC cycles on this machine).
> The preemption timer seems to count faster than what is expected, for
> example only up to 4 million cycles if you set it to one million.
> So, I am leaving the patch out of kvm/queue for now, until I can
> test it on more processors.

I've done some measurements with the help of ftrace on the time it takes
to let the preemption timer trigger (no adjustments via Arthur's patch
were involved): On my Core i7-620M, the preemption timer seems to tick
almost 10 times faster than spec and scale value (5) suggests. I've
loaded a value of 100000, and it took about 130 µs until I got a vmexit
with reason PREEMPTION_TIMER (no other exists in between).

 qemu-system-x86-13765 [003] 298562.966079: bprint:               prepare_vmcs02: preempt val 100000
 qemu-system-x86-13765 [003] 298562.966083: kvm_entry:            vcpu 0
 qemu-system-x86-13765 [003] 298562.966212: kvm_exit:             reason PREEMPTION_TIMER rip 0x401fea info 0 0

That's a frequency of ~769 MHz. The TSC ticks at 2.66 GHz. But 769 MHz *
2^5 is 24.6 GHz. I've read the spec several times, but it seems pretty
clear on this. It just doesn't match reality. Very strange.

Jan



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