[PATCH v2 1/8] x86/fpu: Check for missing AVX and AVX-512 xstate bits
Borislav Petkov
bp at alien8.de
Wed Aug 12 10:48:26 PDT 2026
On Tue, Jul 28, 2026 at 04:47:26PM -0700, Borislav Petkov wrote:
> On Tue, Jul 28, 2026 at 04:34:30PM -0700, Eric Biggers wrote:
> > I don't know what your suggestion is. Are you still asking for the
> > function to be renamed to clear_cpu_caps_xft()? Note that the
> > abbreviation "xft" doesn't appear anywhere else in arch/x86/. Maybe you
> > want the argument inverted? I don't know what you want, sorry.
>
> That's fine - I'll take a look at the final version after having gone through
> the whole pile and then holler if I still see a need.
Ok, here's something ontop of yours. It is tested only in a guest but it
should show the intention.
The first call in:
if (!boot_cpu_has(X86_FEATURE_FPU)) {
is not needed because that is caught earlier:
[ 0.553199] x86/fpu: Giving up, no FPU found and no math emulation present
and we stop there.
The second one can then simply do:
/* Disable all dependent flags too */
setup_clear_cpu_cap(X86_FEATURE_XSAVE);
because that'll clear all dependent flags and we should be good there:
[ 0.560698] do_clear_cpu_cap: clearing 10:0 (xsaveopt)
[ 0.561697] do_clear_cpu_cap: clearing 10:1 (xsavec)
[ 0.562697] do_clear_cpu_cap: clearing 10:3 (xsaves)
[ 0.563697] do_clear_cpu_cap: clearing 4:28 (avx)
[ 0.564697] do_clear_cpu_cap: clearing 16:3 (pku)
[ 0.565697] do_clear_cpu_cap: clearing 9:14 (mpx)
[ 0.566697] do_clear_cpu_cap: clearing 10:2 (xgetbv1)
[ 0.567697] do_clear_cpu_cap: clearing 21:9 ((null))
[ 0.568698] do_clear_cpu_cap: clearing 12:4 (avx_vnni)
[ 0.569697] do_clear_cpu_cap: clearing 4:12 (fma)
[ 0.570697] do_clear_cpu_cap: clearing 16:9 (vaes)
[ 0.571697] do_clear_cpu_cap: clearing 16:10 (vpclmulqdq)
[ 0.572697] do_clear_cpu_cap: clearing 9:5 (avx2)
[ 0.573697] do_clear_cpu_cap: clearing 9:16 (avx512f)
[ 0.574697] do_clear_cpu_cap: clearing 9:21 (avx512ifma)
[ 0.575698] do_clear_cpu_cap: clearing 9:26 (avx512pf)
[ 0.576697] do_clear_cpu_cap: clearing 9:27 (avx512er)
[ 0.577697] do_clear_cpu_cap: clearing 9:28 (avx512cd)
[ 0.578697] do_clear_cpu_cap: clearing 9:17 (avx512dq)
[ 0.579697] do_clear_cpu_cap: clearing 9:30 (avx512bw)
[ 0.580697] do_clear_cpu_cap: clearing 9:31 (avx512vl)
[ 0.581697] do_clear_cpu_cap: clearing 16:1 (avx512vbmi)
[ 0.582697] do_clear_cpu_cap: clearing 16:6 (avx512_vbmi2)
[ 0.583697] do_clear_cpu_cap: clearing 16:11 (avx512_vnni)
[ 0.584697] do_clear_cpu_cap: clearing 16:12 (avx512_bitalg)
[ 0.585697] do_clear_cpu_cap: clearing 18:2 (avx512_4vnniw)
[ 0.586697] do_clear_cpu_cap: clearing 18:3 (avx512_4fmaps)
[ 0.587697] do_clear_cpu_cap: clearing 16:14 (avx512_vpopcntdq)
[ 0.588698] do_clear_cpu_cap: clearing 18:8 (avx512_vp2intersect)
[ 0.589698] do_clear_cpu_cap: clearing 12:5 (avx512_bf16)
[ 0.590697] do_clear_cpu_cap: clearing 18:23 (avx512_fp16)
[ 0.592697] do_clear_cpu_cap: clearing 16:29 (enqcmd)
[ 0.593697] do_clear_cpu_cap: clearing 10:4 ((null))
[ 0.594697] do_clear_cpu_cap: clearing 18:24 (amx_tile)
[ 0.595697] do_clear_cpu_cap: clearing 12:21 ((null))
[ 0.596697] do_clear_cpu_cap: clearing 18:22 (amx_bf16)
[ 0.597697] do_clear_cpu_cap: clearing 18:25 (amx_int8)
[ 0.598697] do_clear_cpu_cap: clearing 16:7 ((null))
and then the rest is streamlined into fpu__init_system_xstate() where all
sanity checking should be performed, as I mentioned intially.
Full diff ontop:
diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index 99801e844b30..96c43ec164f7 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -142,6 +142,8 @@ static void do_clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int feature)
continue;
changed = true;
+ pr_info("%s: clearing %d:%d (%s)\n",
+ __func__, d->feature >> 5, d->feature & 31, x86_cap_flags[d->feature]);
clear_feature(c, d->feature);
}
} while (changed);
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 3c185b2828c7..406ae0841b5f 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -761,23 +761,6 @@ 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.
@@ -785,20 +768,20 @@ static void __init clear_cpu_caps_with_missing_xfeatures(u64 xfeatures)
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;
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);
+ /* Disable all dependent flags too */
+ setup_clear_cpu_cap(X86_FEATURE_XSAVE);
return;
}
@@ -814,7 +797,8 @@ 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
@@ -825,6 +809,22 @@ 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("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("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)) {
/*
@@ -836,8 +836,6 @@ 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;
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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