linux-kernel - Re: [PATCH RFC 1/1] KVM: x86: add param to update master clock periodically

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Date:   Mon, 2 Oct 2023 18:49:06 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Joe Jin <joe.jin@...cle.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com
Subject: Re: [PATCH RFC 1/1] KVM: x86: add param to update master clock periodically

On Mon, Oct 02, 2023, Dongli Zhang wrote:
> > @@ -12185,6 +12203,10 @@ int kvm_arch_hardware_enable(void)
> >  	if (ret != 0)
> >  		return ret;
> >  
> > +	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
> > +		kvm_get_time_scale(NSEC_PER_SEC, tsc_khz * 1000LL,
> > +				   &host_tsc_shift, &host_tsc_to_system_mul);
> 
> I agree that to use the kvmclock to calculate the ns elapsed when updating the
> master clock.
> 
> Would you take the tsc scaling into consideration?
> 
> While the host_tsc_shift and host_tsc_to_system_mul are pre-computed, how about
> the VM using different TSC frequency?

Heh, I'm pretty sure that's completely broken today.  I don't see anything in KVM
that takes hardware TSC scaling into account.

This code:

	if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
		kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,
				   &vcpu->hv_clock.tsc_shift,
				   &vcpu->hv_clock.tsc_to_system_mul);
		vcpu->hw_tsc_khz = tgt_tsc_khz;
		kvm_xen_update_tsc_info(v);
	}

is recomputing the multipler+shift for the current *physical* CPU, it's not
related to the guest's TSC in any way.

__get_kvmclock() again shows that quite clearly, there's no scaling for the guest
TSC anywhere in there.