linux-kernel - Re: [PATCH 07/10] KVM: x86: Avoid NTP frequency skew for KVM clock on 32-bit host

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Message-ID: <b668e938-a48f-4295-a9ec-e66f0abd1bb0@xen.org>
Date: Fri, 19 Apr 2024 16:53:49 +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 07/10] KVM: x86: Avoid NTP frequency skew for KVM clock on
 32-bit host

On 18/04/2024 20:34, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> Commit 53fafdbb8b21 ("KVM: x86: switch KVMCLOCK base to monotonic raw
> clock") did so only for 64-bit hosts, by capturing the boot offset from
> within the existing clocksource notifier update_pvclock_gtod().
> 
> That notifier was added in commit 16e8d74d2da9 ("KVM: x86: notifier for
> clocksource changes") but only on x86_64, because its original purpose
> was just to disable the "master clock" mode which is only supported on
> x86_64.
> 
> Now that the notifier is used for more than disabling master clock mode,
> (well, OK, more than a decade later but clocks are hard), enable it for
> the 32-bit build too so that get_kvmclock_base_ns() can be unaffected by
> NTP sync on 32-bit too.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>   arch/x86/kvm/x86.c | 18 ++++++------------
>   1 file changed, 6 insertions(+), 12 deletions(-)
> 

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