linux-kernel - [PATCH] x86/amd: Work around Erratum 1386 - XSAVES malfunction on context switch

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Message-ID: <20230307174643.1240184-1-andrew.cooper3@citrix.com>
Date:   Tue, 7 Mar 2023 17:46:43 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     LKML <linux-kernel@...r.kernel.org>
CC:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Tavis Ormandy <taviso@...il.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>,
        Alexander Monakov <amonakov@...ras.ru>
Subject: [PATCH] x86/amd: Work around Erratum 1386 - XSAVES malfunction on context switch

AMD Erratum 1386 is summarised as:

  XSAVES Instruction May Fail to Save XMM Registers to the Provided
  State Save Area

This piece of accidental chronomancy causes the %xmm registers to
occasionally reset back to an older value.

Ignore the XSAVES feature on all AMD Zen1/2 hardware.  The XSAVEC
instruction (which works fine) is equivalent on affected parts.

Reported-by: Tavis Ormandy <taviso@...il.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@...rix.com>
---
CC: Thomas Gleixner <tglx@...utronix.de>
CC: Ingo Molnar <mingo@...hat.com>
CC: Borislav Petkov <bp@...en8.de>
CC: Dave Hansen <dave.hansen@...ux.intel.com>
CC: x86@...nel.org
CC: "H. Peter Anvin" <hpa@...or.com>
CC: linux-kernel@...r.kernel.org
CC: Tavis Ormandy <taviso@...il.com>
CC: Alexander Monakov <amonakov@...ras.ru>

Only compile tested.

This wants backporting to all stable trees that understand XSAVES, but
before 5.19(?) needs the XSAVEC support backporting too...
---
 arch/x86/kernel/cpu/amd.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 380753b14cab..f3a4bb479fd5 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -890,6 +890,17 @@ static void init_amd_zn(struct cpuinfo_x86 *c)
 	node_reclaim_distance = 32;
 #endif
 
+	/*
+	 * Work around Erratum 1386.  The XSAVES instruction malfunctions in
+	 * certain circumstances on Zen1/2 uarch, and not all parts have had
+	 * updated microcode at the time of writing (March 2023).
+	 *
+	 * Affected parts all have no supervisor XSAVE states, meaning that
+	 * the XSAVEC instruction (which works fine) is equivelent.
+	 */
+	if (c->x86 == 0x17)
+		clear_cpu_cap(c, X86_FEATURE_XSAVES);
+
 	/* Fix up CPUID bits, but only if not virtualised. */
 	if (!cpu_has(c, X86_FEATURE_HYPERVISOR)) {
 
-- 
2.30.2