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From: Paul Durrant <xadimgnik@gmail.com>
Message-ID: <38a88d9c-4908-4658-a6fa-63ff7a534132@xen.org>
Date: Fri, 19 Apr 2024 16:45:44 +0100
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Subject: Re: [PATCH 05/10] KVM: x86: Explicitly disable TSC scaling without
 CONSTANT_TSC
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>,
 Shuah Khan <shuah@kernel.org>, linux-doc@vger.kernel.org,
 linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
 Oliver Upton <oliver.upton@linux.dev>, Marcelo Tosatti
 <mtosatti@redhat.com>, jalliste@amazon.co.uk, sveith@amazon.de
References: <20240418193528.41780-1-dwmw2@infradead.org>
 <20240418193528.41780-6-dwmw2@infradead.org>
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On 18/04/2024 20:34, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.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@amazon.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@xen.org>