linux-kernel - [PATCH v2] x86: Trust initial offset in architectural TSC-adjust MSRs

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Date: Fri, 19 Apr 2024 16:51:46 +0800
From: Daniel J Blueman <daniel@...ra.org>
To: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Cc: x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	"Peter Zijlstra" <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Daniel J Blueman <daniel@...ra.org>,
	Steffen Persvold <sp@...ascale.com>,
	James Cleverdon <james.cleverdon.external@...den.com>,
	Dimitri Sivanich <sivanich@....com>,
	Steve Wahl <steve.wahl@....com>,
	Prarit Bhargava <prarit@...hat.com>,
	Frank Ramsay <frank.ramsay@....com>,
	Russ Anderson <rja@....com>
Subject: [PATCH v2] x86: Trust initial offset in architectural TSC-adjust MSRs

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.

Signed-off-by: Daniel J Blueman <daniel@...ra.org>
Reviewed-by: Steffen Persvold <sp@...ascale.com>
Reviewed-by: James Cleverdon <james.cleverdon.external@...den.com>
Reviewed-by: Dimitri Sivanich <sivanich@....com>
Cc: Steve Wahl <steve.wahl@....com>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Frank Ramsay <frank.ramsay@....com>
Cc: Russ Anderson <rja@....com>
---
Changes in v2:
- Maintain comment based on feedback
- Rebase against v6.9-rc4

 arch/x86/kernel/tsc_sync.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index 1123ef3ccf90..4334033658ed 100644
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -193,11 +193,9 @@ bool tsc_store_and_check_tsc_adjust(bool bootcpu)
 	cur->warned = false;
 
 	/*
-	 * If a non-zero TSC value for socket 0 may be valid then the default
-	 * adjusted value cannot assumed to be zero either.
+	 * The default adjust value cannot be assumed to be zero on any socket.
 	 */
-	if (tsc_async_resets)
-		cur->adjusted = bootval;
+	cur->adjusted = bootval;
 
 	/*
 	 * Check whether this CPU is the first in a package to come up. In
-- 
2.40.1