linux-kernel - Re: [PATCH 06/10] KVM: x86: Add KVM_VCPU_TSC_SCALE and fix the documentation on TSC migration

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Message-ID: <21bc05cf-0227-4d57-8cfa-81097b95535b@xen.org>
Date: Fri, 19 Apr 2024 16:49:49 +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 06/10] KVM: x86: Add KVM_VCPU_TSC_SCALE and fix the
 documentation on TSC migration

On 18/04/2024 20:34, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> The documentation on TSC migration using KVM_VCPU_TSC_OFFSET is woefully
> inadequate. It ignores TSC scaling, and ignores the fact that the host
> TSC may differ from one host to the next (and in fact because of the way
> the kernel calibrates it, it generally differs from one boot to the next
> even on the same hardware).
> 
> Add KVM_VCPU_TSC_SCALE to extract the actual scale ratio and frac_bits,
> and attempt to document the *awful* process that we're requiring userspace
> to follow to merely preserve the TSC across migration.
> 
> I may have thrown up in my mouth a little when writing that documentation.
> It's an awful API. If we do this, we should be ashamed of ourselves.
> (I also haven't tested the documented process yet).
> 
> Let's use Simon's KVM_VCPU_TSC_VALUE instead.
> https://lore.kernel.org/all/20230202165950.483430-1-sveith@amazon.de/
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>   Documentation/virt/kvm/devices/vcpu.rst | 115 ++++++++++++++++++------
>   arch/x86/include/uapi/asm/kvm.h         |   6 ++
>   arch/x86/kvm/x86.c                      |  15 ++++
>   3 files changed, 109 insertions(+), 27 deletions(-)
> 

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