From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC PATCH v3 2/2] add support for Hyper-V partition reference
 time enlightenment
Date: Thu, 12 Dec 2013 10:34:06 +0100
Message-ID: <52A9830E.4020102@redhat.com>
References: <1386502419-26614-1-git-send-email-vrozenfe@redhat.com> <1386502419-26614-3-git-send-email-vrozenfe@redhat.com> <20131211192700.GA28701@amt.cnet>
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Cc: Vadim Rozenfeld <vrozenfe@redhat.com>, kvm@vger.kernel.org,
	pl@dlhnet.de
To: Marcelo Tosatti <mtosatti@redhat.com>
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Il 11/12/2013 20:27, Marcelo Tosatti ha scritto:
>> > +		if (kvm->arch.hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE) {
>> > +			HV_REFERENCE_TSC_PAGE* tsc_ref;
>> > +			tsc_ref = (HV_REFERENCE_TSC_PAGE*)gfn_to_hva(kvm,
>> > +				kvm->arch.hv_tsc_page >> HV_X64_MSR_TSC_REFERENCE_ADDRESS_SHIFT);
> kvm_read_guest_cached.
> 
>> > +			kvm->arch.hv_ref_time = (((tsc_ref->tsc_scale >> 32) * 
>> > +				native_read_tsc()) >> 32) + tsc_ref->tsc_offset;
> Why native_read_tsc and not ->read_l1_tsc?
> 
> It is easier to trust on the host to check reliability of the TSC: if
> it uses TSC clocksource, then the TSCs are stable. So could condition
> exposing the TSC ref page when ka->use_master_clock=1, see kvm_guest_time_update.
> And hook into pvclock_gtod_notify.
> 
> So in addition to X86_FEATURE_CONSTANT_TSC, check
> ka->use_master_clock=1

FWIW, I agree with all these comments from Marcelo.

Paolo