From: Sean Christopherson <seanjc@google.com>
To: Dongli Zhang <dongli.zhang@oracle.com>
Cc: David Woodhouse <dwmw2@infradead.org>,
Joe Jin <joe.jin@oracle.com>,
x86@kernel.org, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org, pbonzini@redhat.com,
tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
dave.hansen@linux.intel.com
Subject: Re: [PATCH RFC 1/1] KVM: x86: add param to update master clock periodically
Date: Tue, 3 Oct 2023 14:00:06 -0700 [thread overview]
Message-ID: <ZRyA1jSb_Ok9l0po@google.com> (raw)
In-Reply-To: <1326f47a-45c0-963d-d50f-a9774d932744@oracle.com>
On Mon, Oct 02, 2023, Dongli Zhang wrote:
> Hi Sean,
>
> On 10/2/23 18:49, Sean Christopherson wrote:
> > 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.
>
> The below is the code.
>
> line 3175: query freq for current *physical* CPU.
>
> line 3211: scale the freq if scaling is involved.
>
> line 3215: compute the view for guest based on new 'tgt_tsc_khz' after scaling.
>
> 3146 static int kvm_guest_time_update(struct kvm_vcpu *v)
> 3147 {
> 3148 unsigned long flags, tgt_tsc_khz;
> 3149 unsigned seq;
> ... ...
> 3173 /* Keep irq disabled to prevent changes to the clock */
> 3174 local_irq_save(flags);
> 3175 tgt_tsc_khz = get_cpu_tsc_khz();
> ... ...
> 3210 if (kvm_caps.has_tsc_control)
> 3211 tgt_tsc_khz = kvm_scale_tsc(tgt_tsc_khz,
> 3212 v->arch.l1_tsc_scaling_ratio);
> 3213
> 3214 if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
> 3215 kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,
> 3216 &vcpu->hv_clock.tsc_shift,
> 3217 &vcpu->hv_clock.tsc_to_system_mul);
> 3218 vcpu->hw_tsc_khz = tgt_tsc_khz;
> 3219 kvm_xen_update_tsc_info(v);
> 3220 }
>
>
> Would you please let me know if the above understanding is incorrect?
Ah, yeah, you're correct. I missed the call to kvm_scale_tsc() at 3211.
next prev parent reply other threads:[~2023-10-03 21:00 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-26 23:06 [PATCH RFC 1/1] KVM: x86: add param to update master clock periodically Dongli Zhang
2023-09-27 0:29 ` Joe Jin
2023-09-27 0:36 ` Dongli Zhang
2023-09-28 16:18 ` Sean Christopherson
2023-09-29 20:15 ` Dongli Zhang
2023-10-02 8:33 ` David Woodhouse
2023-10-02 16:37 ` Sean Christopherson
2023-10-02 17:17 ` Dongli Zhang
2023-10-02 18:18 ` Sean Christopherson
2023-10-02 21:06 ` Peter Zijlstra
2023-10-02 21:16 ` Peter Zijlstra
2023-10-02 18:16 ` David Woodhouse
2023-10-03 0:53 ` Sean Christopherson
2023-10-03 1:32 ` Dongli Zhang
2023-10-03 1:49 ` Sean Christopherson
2023-10-03 2:07 ` Dongli Zhang
2023-10-03 21:00 ` Sean Christopherson [this message]
2023-10-03 5:54 ` David Woodhouse
2023-10-04 0:04 ` Sean Christopherson
2023-10-04 10:01 ` David Woodhouse
2023-10-04 18:06 ` Sean Christopherson
2023-10-04 19:13 ` Dongli Zhang
2023-10-11 0:20 ` Sean Christopherson
2023-10-11 7:18 ` David Woodhouse
2023-10-13 18:07 ` Sean Christopherson
2023-10-13 18:21 ` David Woodhouse
2023-10-13 19:02 ` Sean Christopherson
2023-10-13 19:12 ` David Woodhouse
2023-10-13 20:03 ` Sean Christopherson
2023-10-13 20:12 ` Dongli Zhang
2023-10-13 23:26 ` Sean Christopherson
2023-10-14 9:49 ` David Woodhouse
2023-10-16 15:47 ` Dongli Zhang
2023-10-16 16:25 ` David Woodhouse
2023-10-16 17:04 ` Dongli Zhang
2023-10-16 18:49 ` Sean Christopherson
2023-10-16 22:04 ` Dongli Zhang
2023-10-16 22:48 ` Sean Christopherson
2023-10-17 16:18 ` Dongli Zhang
2023-10-03 9:12 ` David Woodhouse
2023-10-04 0:07 ` Sean Christopherson
2023-10-04 8:06 ` David Woodhouse
2023-10-03 14:29 ` David Woodhouse
2023-10-04 0:10 ` Sean Christopherson
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