linux-kernel - [PATCH v2] x86/CPU/AMD: Prevent reset reasons from being retained among boots

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Message-ID: <20251010165959.49737-1-i@rong.moe>
Date: Sat, 11 Oct 2025 00:59:58 +0800
From: Rong Zhang <i@...g.moe>
To: 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
Cc: Rong Zhang <i@...g.moe>,
	"H. Peter Anvin" <hpa@...or.com>,
	Mario Limonciello <mario.limonciello@....com>,
	Yazen Ghannam <yazen.ghannam@....com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2] x86/CPU/AMD: Prevent reset reasons from being retained among boots

The S5_RESET_STATUS register is parsed on boot and printed to kmsg.
However, this could sometimes be misleading and lead to users wasting a
lot of time on meaningless debugging for two reasons:

* Some bits are never cleared by hardware. It's the software's
responsibility to clear them as per the Processor Programming Reference
(see Link:).

* Some rare hardware-initiated platform resets do not update the
register at all.

In both cases, a previous reboot could leave its trace in the register,
resulting in users seeing unrelated reboot reasons while debugging
random reboots afterward.

Clearing all reason bits solves the issue. Since all reason bits are
write-1-to-clear and we must preserve all other bits, this is done by
writing the read value back to the register.

Fixes: ab8131028710 ("x86/CPU/AMD: Print the reason for the last reset")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537#attach_303991
Signed-off-by: Rong Zhang <i@...g.moe>
---
Changes in v2:
- Remove a debug message (suggested by Borislav Petkov)
- No longer mention this behavior in the documentation
- Link to v1: https://lore.kernel.org/r/20250913144245.23237-1-i@rong.moe/
---
 arch/x86/kernel/cpu/amd.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 5398db4dedb4a..ccaa51ce63f6e 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1355,11 +1355,23 @@ static __init int print_s5_reset_status_mmio(void)
 		return 0;
 
 	value = ioread32(addr);
-	iounmap(addr);
 
 	/* Value with "all bits set" is an error response and should be ignored. */
-	if (value == U32_MAX)
+	if (value == U32_MAX) {
+		iounmap(addr);
 		return 0;
+	}
+
+	/*
+	 * Clear all reason bits so they won't be retained if the next reset
+	 * does not update the register. Besides, some bits are never cleared by
+	 * hardware so it's software's responsibility to clear them.
+	 *
+	 * Writing the value back effectively clears all reason bits as they are
+	 * write-1-to-clear.
+	 */
+	iowrite32(value, addr);
+	iounmap(addr);
 
 	for (i = 0; i < ARRAY_SIZE(s5_reset_reason_txt); i++) {
 		if (!(value & BIT(i)))

base-commit: 7d24c651ce163bc04e7683ec75bf976b6ff042e2
-- 
2.51.0