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From: Paul Durrant <xadimgnik@gmail.com>
Message-ID: <3776d48b-652e-409a-b991-d96d6a9e777e@xen.org>
Date: Fri, 24 May 2024 15:16:20 +0100
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Subject: Re: [RFC PATCH v3 18/21] KVM: x86: Avoid gratuitous global clock
 reload in kvm_arch_vcpu_load()
To: David Woodhouse <dwmw2@infradead.org>, kvm@vger.kernel.org
Cc: Paolo Bonzini <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
 Sean Christopherson <seanjc@google.com>, Thomas Gleixner
 <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
 x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
 Peter Zijlstra <peterz@infradead.org>, Juri Lelli <juri.lelli@redhat.com>,
 Vincent Guittot <vincent.guittot@linaro.org>,
 Dietmar Eggemann <dietmar.eggemann@arm.com>,
 Steven Rostedt <rostedt@goodmis.org>, Ben Segall <bsegall@google.com>,
 Mel Gorman <mgorman@suse.de>, Daniel Bristot de Oliveira
 <bristot@redhat.com>, Valentin Schneider <vschneid@redhat.com>,
 Shuah Khan <shuah@kernel.org>, linux-doc@vger.kernel.org,
 linux-kernel@vger.kernel.org, jalliste@amazon.co.uk, sveith@amazon.de,
 zide.chen@intel.com, Dongli Zhang <dongli.zhang@oracle.com>,
 Chenyi Qiang <chenyi.qiang@intel.com>
References: <20240522001817.619072-1-dwmw2@infradead.org>
 <20240522001817.619072-19-dwmw2@infradead.org>
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On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> Commit d98d07ca7e034 ("KVM: x86: update pvclock area conditionally, on
> cpu migration") turned an unconditional KVM_REQ_CLOCK_UPDATE into a
> conditional one, if either the master clock isn't enabled *or* the vCPU
> was not previously scheduled (vcpu->cpu == -1). The commit message doesn't
> explain the latter condition, which is specifically for the master clock
> case.
> 
> Commit 0061d53daf26f ("KVM: x86: limit difference between kvmclock
> updates") later turned that into a KVM_REQ_GLOBAL_CLOCK_UPDATE to avoid
> skew between vCPUs.
> 
> In master clock mode there is no need for any of that, regardless of
> whether/where this vCPU was previously scheduled.
> 
> Do it only if (!kvm->arch.use_master_clock).
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>   arch/x86/kvm/x86.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 

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