linux-kernel - [PATCH AUTOSEL 5.4 2/5] x86/cpu: Don't clear X86_FEATURE_LAHF_LM flag in init_amd_k8() on AMD when running in a virtual machine

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Message-Id: <20250331143728.1686696-2-sashal@kernel.org>
Date: Mon, 31 Mar 2025 10:37:23 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Max Grobecker <max@...becker.info>,
	Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sashal@...nel.org>,
	tglx@...utronix.de,
	mingo@...hat.com,
	dave.hansen@...ux.intel.com,
	x86@...nel.org,
	thomas.lendacky@....com,
	riel@...riel.com,
	perry.yuan@....com,
	mario.limonciello@....com,
	seanjc@...gle.com,
	darwi@...utronix.de,
	mjguzik@...il.com
Subject: [PATCH AUTOSEL 5.4 2/5] x86/cpu: Don't clear X86_FEATURE_LAHF_LM flag in init_amd_k8() on AMD when running in a virtual machine

From: Max Grobecker <max@...becker.info>

[ Upstream commit a4248ee16f411ac1ea7dfab228a6659b111e3d65 ]

When running in a virtual machine, we might see the original hardware CPU
vendor string (i.e. "AuthenticAMD"), but a model and family ID set by the
hypervisor. In case we run on AMD hardware and the hypervisor sets a model
ID < 0x14, the LAHF cpu feature is eliminated from the the list of CPU
capabilities present to circumvent a bug with some BIOSes in conjunction with
AMD K8 processors.

Parsing the flags list from /proc/cpuinfo seems to be happening mostly in
bash scripts and prebuilt Docker containers, as it does not need to have
additionals tools present – even though more reliable ways like using "kcpuid",
which calls the CPUID instruction instead of parsing a list, should be preferred.
Scripts, that use /proc/cpuinfo to determine if the current CPU is
"compliant" with defined microarchitecture levels like x86-64-v2 will falsely
claim the CPU is incapable of modern CPU instructions when "lahf_lm" is missing
in that flags list.

This can prevent some docker containers from starting or build scripts to create
unoptimized binaries.

Admittably, this is more a small inconvenience than a severe bug in the kernel
and the shoddy scripts that rely on parsing /proc/cpuinfo
should be fixed instead.

This patch adds an additional check to see if we're running inside a
virtual machine (X86_FEATURE_HYPERVISOR is present), which, to my
understanding, can't be present on a real K8 processor as it was introduced
only with the later/other Athlon64 models.

Example output with the "lahf_lm" flag missing in the flags list
(should be shown between "hypervisor" and "abm"):

    $ cat /proc/cpuinfo
    processor       : 0
    vendor_id       : AuthenticAMD
    cpu family      : 15
    model           : 6
    model name      : Common KVM processor
    stepping        : 1
    microcode       : 0x1000065
    cpu MHz         : 2599.998
    cache size      : 512 KB
    physical id     : 0
    siblings        : 1
    core id         : 0
    cpu cores       : 1
    apicid          : 0
    initial apicid  : 0
    fpu             : yes
    fpu_exception   : yes
    cpuid level     : 13
    wp              : yes
    flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca
                      cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx rdtscp
                      lm rep_good nopl cpuid extd_apicid tsc_known_freq pni
                      pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic movbe popcnt
                      tsc_deadline_timer aes xsave avx f16c hypervisor abm
                      3dnowprefetch vmmcall bmi1 avx2 bmi2 xsaveopt

... while kcpuid shows the feature to be present in the CPU:

    # kcpuid -d | grep lahf
         lahf_lm             - LAHF/SAHF available in 64-bit mode

[ mingo: Updated the comment a bit, incorporated Boris's review feedback. ]

Signed-off-by: Max Grobecker <max@...becker.info>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org
Cc: Borislav Petkov <bp@...en8.de>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/kernel/cpu/amd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 533451498c8f8..9bcc099484c89 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -808,7 +808,7 @@ static void init_amd_k8(struct cpuinfo_x86 *c)
 	 * (model = 0x14) and later actually support it.
 	 * (AMD Erratum #110, docId: 25759).
 	 */
-	if (c->x86_model < 0x14 && cpu_has(c, X86_FEATURE_LAHF_LM)) {
+	if (c->x86_model < 0x14 && cpu_has(c, X86_FEATURE_LAHF_LM) && !cpu_has(c, X86_FEATURE_HYPERVISOR)) {
 		clear_cpu_cap(c, X86_FEATURE_LAHF_LM);
 		if (!rdmsrl_amd_safe(0xc001100d, &value)) {
 			value &= ~BIT_64(32);
-- 
2.39.5