[PATCH v2 1/8] x86/fpu: Check for missing AVX and AVX-512 xstate bits
From: Eric Biggers <ebiggers@kernel.org>
Date: 2026-07-28 02:19:17
Also in:
linux-crypto, linux-um, lkml
Subsystem:
the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
If the CPU declares AVX or AVX-512 support, verify that the corresponding xstate bits are also set. If not, warn and clear them. This eliminates the perceived need for AVX and AVX-512 optimized code in the kernel to call cpu_has_xfeatures(). That has never been universally done, which strongly suggests that it has never really been needed in practice, but this should remove any remaining doubt. Signed-off-by: Eric Biggers <ebiggers@kernel.org> --- arch/x86/kernel/fpu/xstate.c | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+)
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index a7b6524a9dea2..904ff933c0d88 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c@@ -799,6 +799,23 @@ static u64 __init guest_default_mask(void) return ~(u64)XFEATURE_MASK_USER_DYNAMIC; } +/* Clear any X86_FEATURE_* used by the kernel whose xfeatures are missing. */ +static void __init clear_cpu_caps_with_missing_xfeatures(u64 xfeatures) +{ + u64 mask; + + mask = XFEATURE_MASK_FPSSE | XFEATURE_MASK_YMM; + if (boot_cpu_has(X86_FEATURE_AVX) && (xfeatures & mask) != mask) { + pr_err("x86/fpu: Disabling AVX support due to missing xstate features\n"); + setup_clear_cpu_cap(X86_FEATURE_AVX); + } + mask = XFEATURE_MASK_FPSSE | XFEATURE_MASK_YMM | XFEATURE_MASK_AVX512; + if (boot_cpu_has(X86_FEATURE_AVX512F) && (xfeatures & mask) != mask) { + pr_err("x86/fpu: Disabling AVX-512 support due to missing xstate features\n"); + setup_clear_cpu_cap(X86_FEATURE_AVX512F); + } +} + /* * Enable and initialize the xsave feature. * Called once per system bootup.
@@ -812,12 +829,14 @@ void __init fpu__init_system_xstate(unsigned int legacy_size) if (!boot_cpu_has(X86_FEATURE_FPU)) { pr_info("x86/fpu: No FPU detected\n"); + clear_cpu_caps_with_missing_xfeatures(0); return; } if (!boot_cpu_has(X86_FEATURE_XSAVE)) { pr_info("x86/fpu: x87 FPU will use %s\n", boot_cpu_has(X86_FEATURE_FXSR) ? "FXSAVE" : "FSAVE"); + clear_cpu_caps_with_missing_xfeatures(0); return; }
@@ -855,6 +874,8 @@ void __init fpu__init_system_xstate(unsigned int legacy_size) goto out_disable; } + clear_cpu_caps_with_missing_xfeatures(fpu_kernel_cfg.max_features); + fpu_kernel_cfg.independent_features = fpu_kernel_cfg.max_features & XFEATURE_MASK_INDEPENDENT;
--
2.55.0