linux-kernel - Re: [PATCH] KVM: x86/xen: improve accuracy of Xen timers

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Message-ID: <a2a13598-6d37-47a2-8a13-9963acb0f1e5@xen.org>
Date:   Tue, 31 Oct 2023 12:22:08 +0000
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Paul Durrant <xadimgnik@...il.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] KVM: x86/xen: improve accuracy of Xen timers

On 31/10/2023 12:11, David Woodhouse wrote:
> 
> 
> On 31 October 2023 12:06:17 GMT, Paul Durrant <xadimgnik@...il.com> wrote:
>> On 31/10/2023 11:42, David Woodhouse wrote:
>>> Secondly, it's also wrong thing to do in the general case. Let's say KVM does its thing and snaps the kvmclock backwards in time on a KVM_REQ_CLOCK_UPDATE... do we really want to reinterpret existing timers against the new kvmclock? They were best left alone, I think.
>>
>> Do we not want to do exactly that? If the master clock is changed, why would we not want to re-interpret the guest's idea of time? That update will be visible to the guest when it re-reads the PV clock source.
> 
> Well no, because the guest set that timer *before* we yanked the clock from under it, and probably wants it interpreted in the time scale which was in force at the time it was set.
> 
> But more to the point, KVM shouldn't be doing that! We need to *fix* the kvmclock brokenness, not design further band-aids around it.

Ok, fair enough.

> 
> As I said, this patch stands even *after* we fix kvmclock, because it handles the timer delta calculation from an single TSC read.
> 
> But overengineering a timer reset on KVM_REQ_CLOCK_UPDATE would not.

I'm not sure what you intend to do to kvmlock, so not sure whether we'll 
still need the __pvclock_read_cycles(&vcpu->arch.hv_clock, guest_tsc) 
but this patch (with the extra check on validity of hv_clock) does fix 
the drift so...

Reviewed-by: Paul Durrant <paul@....org>