linux-kernel - Re: [PATCH 09/10] KVM: x86: Kill KVM_REQ_GLOBAL_CLOCK

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Date: Fri, 19 Apr 2024 16:57:47 +0100
From: Paul Durrant <xadimgnik@...il.com>
To: David Woodhouse <dwmw2@...radead.org>, kvm@...r.kernel.org
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Sean Christopherson <seanjc@...gle.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>,
 Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 Oliver Upton <oliver.upton@...ux.dev>, Marcelo Tosatti
 <mtosatti@...hat.com>, jalliste@...zon.co.uk, sveith@...zon.de
Subject: Re: [PATCH 09/10] KVM: x86: Kill KVM_REQ_GLOBAL_CLOCK_UPDATE

On 18/04/2024 20:34, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> This was introduced in commit 0061d53daf26 ("KVM: x86: limit difference
> between kvmclock updates") to reduce cross-vCPU differences which arose
> because the KVM clock was based on CLOCK_MONOTONIC and thus subject to
> NTP frequency corrections.
> 
> However, commit 53fafdbb8b21 ("KVM: x86: switch KVMCLOCK base to
> monotonic raw clock") switched to using CLOCK_MONOTONIC_RAW as the basis
> for the KVM clock, avoiding the NTP frequency skew altogether.
> 
> So remove KVM_REQ_GLOBAL_CLOCK_UPDATE. In kvm_write_system_time(), all
> that's needed is a single KVM_REQ_CLOCK_UPDATE for the vCPU whose KVM
> clock is being configured.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>   arch/x86/include/asm/kvm_host.h |  2 +-
>   arch/x86/kvm/x86.c              | 57 ++-------------------------------
>   2 files changed, 4 insertions(+), 55 deletions(-)
> 

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