[PATCH v3 Bugfix 1/6] x86/xsave.c: Fix xstate offsets and sizes enumeration

From: Fenghua Yu
Date: Fri May 08 2015 - 17:35:17 EST

From: Fenghua Yu <fenghua.yu@xxxxxxxxx>

When enumerating xstate offsets and sizes from cpuid (eax=0x0d, ecx>=2),
it's possible that state m is not implemented while state n (n>m)
is implemented. So enumeration shouldn't stop at state m.

There is no platform configured like above yet. But this could be a problem
in the future. For example, suppose XCR0=0xe7, that means FP, SSE, AVX, and
AVX-512 states are enabled and MPX states (bit 3 and 4) are not enabled.
Then in setup_xstate_features(), after finding BNDREGS size is 0 (i.e. eax
from CPUID xstate subleaf 3, break from the for loop. That stops finding
xstate_offsets and xstate_sizes for AVX-512. Later on incorrect
xstate_offsets and xstate_sizes for AVX-512 will be used in a few places
and will causes issues.

This patch enumerates xstate offsets and sizes for all kernel supported
xstates. If a state is not implemented in hardware or not enabled in XCR0,
its size is set as zero and its offset is read from cpuid.

Ingo is rewriting fpu/xstate and his big patchset includes this patch at:

I still send this patch in the patchset because we need to fix this bug
in upstream ASAP.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxx>
arch/x86/kernel/xsave.c | 11 +++--------
1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index 87a815b..3c0a9d1 100644
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -465,23 +465,18 @@ static inline void xstate_enable(void)
static void __init setup_xstate_features(void)
- int eax, ebx, ecx, edx, leaf = 0x2;
+ int eax, ebx, ecx, edx, leaf;

xstate_features = fls64(pcntxt_mask);
xstate_offsets = alloc_bootmem(xstate_features * sizeof(int));
xstate_sizes = alloc_bootmem(xstate_features * sizeof(int));

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

- if (eax == 0)
- break;
xstate_offsets[leaf] = ebx;
xstate_sizes[leaf] = eax;
- leaf++;
- } while (1);
+ }


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