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: <c2dfb1cb3947c3cb2e62973c40cd0880d779d26e.camel@infradead.org>
Date: Fri, 19 Apr 2024 16:53:25 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: paul@....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 Fri, 2024-04-19 at 16:49 +0100, Paul Durrant wrote:
> 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/

Oops, I meant to change that commit message. I've changed my mind, as
KVM_VCPU_TSC_VALUE sets the TSC using the KVM clock as a reference...
which is utterly backwards, since the KVM clock is *defined* in terms
of the TSC.

Will do so before posting it again.

May also include the "loop over KVM_GET_CLOCK until tai_offset didn't
change" part to fix the leap seconbd problem.

I think we do need to implement a better way to get { TAI, host TSC }
than abusing KVM_GET_CLOCK when we don't even *care* about the KVM
clock. We need a way to migrate the arch counters on platforms other
than x86 too. But I'm trying to leave that part for later. At least on
x86 we *do* have a way to migrate the TSC accurately, even if it's
awful.

> > 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>
> 

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