linux-kernel - Re: [PATCH v3 09/14] x86/svm: Deflake svm_tsc_scale

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Message-ID: <fqzv25ycu2tpvg7ljfsl7mzqulu7qkjc2vqjdh6hxhyn4svrd5@46fo2mlsysyx>
Date: Fri, 14 Nov 2025 05:46:37 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Kevin Cheng <chengkev@...gle.com>, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 09/14] x86/svm: Deflake svm_tsc_scale_test

On Thu, Nov 13, 2025 at 04:34:35PM -0800, Sean Christopherson wrote:
> On Mon, Nov 10, 2025, Yosry Ahmed wrote:
> > On an AMT Turin (EPYC Zen 5), svm_tsc_scale_test flakes on the last test
> > case with 0.0001 TSC scaling ratio, even with the 24-bit shift for
> > stability. On failure, the actual value is 49 instead of the expected
> > 50.
> > 
> > Use a higher scaling ratio, 0.001, which makes the test pass
> > consistently.
> 
> Top-tier analysis right here :-D

Not my proudest moment :P

I saw the test is already using some arbitrary numbers and I was too
lazy to do what Jim did. I initially had a patch that allows for a
certain % error instead, like the selftest, but I opted for the simple
change :P

In my defense, I did call it not-so-sophisticated in the cover letter!

> 
> I'm going to take Jim's version instead of papering over the bug.
> 
> https://lore.kernel.org/all/CALMp9eQep3H-OtqmLe3O2MsOT-Vx4y0-LordKgN+pkp04VLSWw@mail.gmail.com

I am actually glad there's a better patch than mine :)