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From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: rdtsc() in kvm-unit-tests on x86
Date: Mon, 10 Aug 2015 16:47:44 +0200
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Cc: Christoffer Dall <christofferdall@gmail.com>,
	KVM General <kvm@vger.kernel.org>,
	Shih-Wei Li <shihwei@cs.columbia.edu>
To: Jintack Lim <jintack@cs.columbia.edu>
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On 10/08/2015 16:14, Jintack Lim wrote:
>> > Yes, you just use the TSC. :)  However, you first have to check that the
>> > TSC is consistent across CPUs.  On older machines it's not, but the
>> > kernel can detect it.
> Thanks, Paolo.
> 
> What would be the best way to check if TSC is consistent across CPUs?

You need to have boot_cpu_has(X86_FEATURE_CONSTANT_TSC) and
boot_cpu_has(X86_FEATURE_NONSTOP_TSC).

However, I would just use TAI (ktime_get_clocktai).  x86 KVM provides a
paravirtual interface that synchronizes CLOCK_TAI with the host, and
using it is the simplest way to get synchronized times between the host
and the guest.

Paolo

> Is it synchronized in nano second (or even cpu clock cycle) resolution?
> 
> To get synchronized tsc across the host and the guest,
> just calling rdtscll() in host and guest would be enough?