linux-kernel - Re: [PATCH v2] KVM: x86: Refine calculation of guest wall clock to use a single TSC read

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Date:   Thu,  5 Oct 2023 20:48:45 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org,
        David Woodhouse <dwmw2@...radead.org>
Cc:     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>, Paul Durrant <paul@....org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: x86: Refine calculation of guest wall clock to
 use a single TSC read

On Thu, 05 Oct 2023 10:16:10 +0100, David Woodhouse wrote:
> When populating the guest's PV wall clock information, KVM currently does
> a simple 'kvm_get_real_ns() - get_kvmclock_ns(kvm)'. This is an antipattern
> which should be avoided; when working with the relationship between two
> clocks, it's never correct to obtain one of them "now" and then the other
> at a slightly different "now" after an unspecified period of preemption
> (which might not even be under the control of the kernel, if this is an
> L1 hosting an L2 guest under nested virtualization).
> 
> [...]

Applied to kvm-x86 misc, thanks!  And double thanks for the function comment,
it's awesome!

Note, I moved the declaration in x86.h just below get_kvmclock_ns() to bundle
the two kvmclock helpers together.

[1/1] KVM: x86: Refine calculation of guest wall clock to use a single TSC read
      https://github.com/kvm-x86/linux/commit/5d6d6a7d7e66

--
https://github.com/kvm-x86/linux/tree/next