[PATCH 1/8] x86/fpu: Check for missing AVX and AVX-512 xstate bits
From: Eric Biggers <ebiggers@kernel.org>
Date: 2026-06-26 04:39:24
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 | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+)
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index a7b6524a9dea..7f7e62e4ebc5 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c@@ -797,10 +797,27 @@ static u64 __init guest_default_mask(void) * for KVM guests. */ 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. */ void __init fpu__init_system_xstate(unsigned int legacy_size)
@@ -853,10 +870,12 @@ void __init fpu__init_system_xstate(unsigned int legacy_size) pr_err("x86/fpu: Both APX/MPX present in the CPU's xstate features: 0x%llx.\n", fpu_kernel_cfg.max_features); 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; /* * Clear XSAVE features that are disabled in the normal CPUID.
--
2.54.0