[PATCH 1/8] x86/fpu: Check for missing AVX and AVX-512 xstate bits

[Eric Biggers]

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>
---
 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