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Subject[PATCH 200/208] x86/fpu/xstate: Don't assume the first zero xfeatures zero bit means the end
The current xstate code in setup_xstate_features() assumes that
the first zero bit means the end of xfeatures - but that is not
so, the SDM clearly states that an arbitrary set of xfeatures
might be enabled - and it is also clear from the description
of the compaction feature that holes are possible:


Compacted format. Each state component i (i ≥ 2) is located at a byte
offset from the base address of the XSAVE area based on the XCOMP_BV
field in the XSAVE header:

— If XCOMP_BV[i] = 0, state component i is not in the XSAVE area.

— If XCOMP_BV[i] = 1, the following items apply:

• If XCOMP_BV[j] = 0 for every j, 2 ≤ j < i, state component i is
located at a byte offset 576 from the base address of the XSAVE
area. (This item applies if i is the first bit set in bits 62:2 of
the XCOMP_BV; it implies that state component i is located at the
beginning of the extended region.)

• Otherwise, let j, 2 ≤ j < i, be the greatest value such that
XCOMP_BV[j] = 1. Then state component i is located at a byte offset
X from the location of state component j, where X is the number of
bytes required for state component j as enumerated in
CPUID.(EAX=0DH,ECX=j):EAX. (This item implies that state component i
immediately follows the preceding state component whose bit is set
in XCOMP_BV.)"

So don't assume that the first zero xfeatures bit means the end of
all xfeatures - iterate through all of them.

I'm not aware of hardware that triggers this currently.

Cc: Andy Lutomirski <>
Cc: Borislav Petkov <>
Cc: Dave Hansen <>
Cc: Fenghua Yu <>
Cc: H. Peter Anvin <>
Cc: Linus Torvalds <>
Cc: Oleg Nesterov <>
Cc: Thomas Gleixner <>
Signed-off-by: Ingo Molnar <>
arch/x86/kernel/fpu/xstate.c | 17 +++++++++--------
1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index cd7f1a6bd933..a024fa591a93 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -168,26 +168,27 @@ void fpu__init_cpu_xstate(void)

- * Record the offsets and sizes of different state managed by the xsave
- * memory layout.
+ * Record the offsets and sizes of various xstates contained
+ * in the XSAVE state memory layout.
+ *
+ * ( Note that certain features might be non-present, for them
+ * we'll have 0 offset and 0 size. )
static void __init setup_xstate_features(void)
- int eax, ebx, ecx, edx, leaf = 0x2;
+ u32 eax, ebx, ecx, edx, leaf;

xfeatures_nr = fls64(xfeatures_mask);

- do {
+ for (leaf = 2; leaf < xfeatures_nr; leaf++) {
cpuid_count(XSTATE_CPUID, leaf, &eax, &ebx, &ecx, &edx);

- if (eax == 0)
- break;
xstate_offsets[leaf] = ebx;
xstate_sizes[leaf] = eax;

+ printk(KERN_INFO "x86/fpu: xstate_offset[%d]: %04x, xstate_sizes[%d]: %04x\n", leaf, ebx, leaf, eax);
- } while (1);
+ }

static void print_xstate_feature(u64 xstate_mask)

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