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Date:   Tue, 03 Oct 2023 10:12:09 +0100
From:   David Woodhouse <dwmw2@infradead.org>
To:     Sean Christopherson <seanjc@google.com>
CC:     Dongli Zhang <dongli.zhang@oracle.com>,
        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: =?US-ASCII?Q?Re=3A_=5BPATCH_RFC_1/1=5D_KVM=3A_x86=3A_add_par?= =?US-ASCII?Q?am_to_update_master_clock_periodically?=
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On 3 October 2023 01:53:11 BST, Sean Christopherson <seanjc@google=2Ecom> =
wrote:
>I think there is still use for synchronizing with the host's view of time=
, e=2Eg=2E
>to deal with lost time across host suspend+resume=2E
>
>So I don't think we can completely sever KVM's paravirt clocks from host =
time,
>at least not without harming use cases that rely on the host's view to ke=
ep
>accurate time=2E  And honestly at that point, the right answer would be t=
o stop
>advertising paravirt clocks entirely=2E
>
>But I do think we can address the issues that Dongli and David are obvers=
ing
>where guest time drifts even though the host kernel's base time hasn't ch=
anged=2E
>If I've pieced everything together correctly, the drift can be eliminated=
 simply
>by using the paravirt clock algorithm when converting the delta from the =
raw TSC
>to nanoseconds=2E
>
>This is *very* lightly tested, as in it compiles and doesn't explode, but=
 that's
>about all I've tested=2E

Hm, I don't think I like this=2E

You're making get_monotonic_raw() not *actually* return the monotonic_raw =
clock, but basically return the kvmclock instead? And why? So that when KVM=
 attempts to synchronize the kvmclock to the monotonic_raw clock, it gets t=
ricked into actually synchronizing the kvmclock to *itself*?

If you get this right, don't we have a fairly complex piece of code that h=
as precisely *no* effect?=20

Can't we just *refrain* from synchronizing the kvmclock to *anything*, in =
the CONSTANT_TSC case? Why do we do that anyway?

(Suspend/resume, live update and live migration are different=2E In *those=
* cases we may need to preserve both the guest TSC and kvmclock based on ei=
ther the host TSC or CLOCK_TAI=2E But that's different=2E)