linux-kernel - Re: [PATCH 05/10] KVM: x86: Explicitly disable TSC scaling without CONSTANT

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Date: Fri, 19 Apr 2024 16:45:44 +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 05/10] KVM: x86: Explicitly disable TSC scaling without
 CONSTANT_TSC

On 18/04/2024 20:34, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> KVM does make an attempt to cope with non-constant TSC, and has notifiers
> to handle host TSC frequency changes. However, it *only* adjusts the KVM
> clock, and doesn't adjust TSC frequency scaling when the host changes.
> 
> This is presumably because non-constant TSCs were fixed in hardware long
> before TSC scaling was implemented, so there should never be real CPUs
> which have TSC scaling but *not* CONSTANT_TSC.
> 
> Such a combination could potentially happen in some odd L1 nesting
> environment, but it isn't worth trying to support it. Just make the
> dependency explicit.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>   arch/x86/kvm/svm/svm.c | 3 ++-
>   arch/x86/kvm/vmx/vmx.c | 2 +-
>   2 files changed, 3 insertions(+), 2 deletions(-)
> 

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