[tip: x86/fpu] x86/fpu: Check for missing AVX and AVX-512 xstate bits

[tip-bot2 for Eric Biggers]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     518aa9948e4f2ba69b7066371d3ac8e4d31d604e
Gitweb:        https://git.kernel.org/tip/518aa9948e4f2ba69b7066371d3ac8e4d31d604e
Author:        Eric Biggers <ebiggers@xxxxxxxxxx>
AuthorDate:    Tue, 01 Sep 2026 07:57:33 -07:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Wed, 02 Sep 2026 10:35:40 -07:00

x86/fpu: Check for missing AVX and AVX-512 xstate bits

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@xxxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Acked-by: Christoph Hellwig <hch@xxxxxx>
Link: https://patch.msgid.link/20260901145740.42337-2-ebiggers@xxxxxxxxxx
---
 arch/x86/kernel/fpu/xstate.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index a7b6524..97cfd4f 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -806,7 +806,7 @@ static u64 __init guest_default_mask(void)
 void __init fpu__init_system_xstate(unsigned int legacy_size)
 {
 	unsigned int eax, ebx, ecx, edx;
-	u64 xfeatures;
+	u64 xfeatures, mask;
 	int err;
 	int i;
 
@@ -818,6 +818,8 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 	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");
+		/* Disable all dependent flags too */
+		setup_clear_cpu_cap(X86_FEATURE_XSAVE);
 		return;
 	}
 
@@ -833,7 +835,8 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 	cpuid_count(CPUID_LEAF_XSTATE, 1, &eax, &ebx, &ecx, &edx);
 	fpu_kernel_cfg.max_features |= ecx + ((u64)edx << 32);
 
-	if ((fpu_kernel_cfg.max_features & XFEATURE_MASK_FPSSE) != XFEATURE_MASK_FPSSE) {
+	mask = XFEATURE_MASK_FPSSE;
+	if ((fpu_kernel_cfg.max_features & mask) != mask) {
 		/*
 		 * This indicates that something really unexpected happened
 		 * with the enumeration.  Disable XSAVE and try to continue
@@ -844,6 +847,24 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 		goto out_disable;
 	}
 
+	mask |= XFEATURE_MASK_YMM;
+	if (boot_cpu_has(X86_FEATURE_AVX)) {
+		if ((fpu_kernel_cfg.max_features & mask) != mask) {
+			pr_err(FW_BUG
+			       "x86/fpu: Disabling AVX support due to missing xstate features\n");
+			setup_clear_cpu_cap(X86_FEATURE_AVX);
+		}
+	}
+
+	mask |= XFEATURE_MASK_AVX512;
+	if (boot_cpu_has(X86_FEATURE_AVX512F)) {
+		if ((fpu_kernel_cfg.max_features & mask) != mask) {
+			pr_err(FW_BUG
+			       "x86/fpu: Disabling AVX-512 support due to missing xstate features\n");
+			setup_clear_cpu_cap(X86_FEATURE_AVX512F);
+		}
+	}
+
 	if (fpu_kernel_cfg.max_features & XFEATURE_MASK_APX &&
 	    fpu_kernel_cfg.max_features & (XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR)) {
 		/*