linux-kernel - Re: [PATCH] x86: Trust initial offset in architectural TSC-adjust MSRs

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Date: Mon, 15 Apr 2024 09:22:08 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Daniel J Blueman <daniel@...ra.org>
Cc: 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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Trust initial offset in architectural TSC-adjust
 MSRs

On Mon, Feb 26, 2024 at 03:25:33PM +0800, Daniel J Blueman wrote:
> When the BIOS configures the architectural TSC-adjust MSRs on secondary
> sockets to correct a constant inter-chassis offset, after Linux brings
> the cores online, the TSC sync check later resets the core-local MSR to
> 0, triggering HPET fallback and leading to performance loss.
> 
> Fix this by unconditionally using the initial adjust values read from the
> MSRs. Trusting the initial offsets in this architectural mechanism is a
> better approach than special-casing workarounds for specific platforms.

Given the amount of BIOS fail in general, I'm thinking the number of
machines that have multi-chassis and get this right are far less than
the 'small' systems that get this wrong.