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Subject[PATCH v3] x86/fpu: copy MXCSR & MXCSR_FLAGS with SSE/YMM state
On Fri, 10 Feb 2017 09:00:54 +0100
Ingo Molnar <mingo@kernel.org> wrote:

> > /* Copy both mxcsr & mxcsr_flags */
> > #define MXCSR_AND_FLAGS_SIZE sizeof(u64)
>
> Yeah, that define would make it pretty clear what's going on. Please make it a bit
> more vebose:
>
> /* Copy both mxcsr & mxcsr_flags with a single u64 memcpy: */
> #define MXCSR_AND_FLAGS_SIZE sizeof(u64)

OK, here it is :)

---8<---
Subject: x86/fpu: copy MXCSR & MXCSR_FLAGS with SSE/YMM state

On Skylake CPUs I noticed that XRSTOR is unable to deal with states
created by copyout_from_xsaves if the xstate has only SSE/YMM state, and
no FP state. That is, xfeatures had XFEATURE_MASK_SSE set, but not
XFEATURE_MASK_FP.

The reason is that part of the SSE/YMM state lives in the MXCSR and
MXCSR_FLAGS fields of the FP state.

Ensure that whenever we copy SSE or YMM state around, the MXCSR and
MXCSR_FLAGS fields are also copied around.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
arch/x86/include/asm/fpu/types.h | 3 +++
arch/x86/kernel/fpu/xstate.c | 44 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 47 insertions(+)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index d15cbfe0e8c4..ea65ab22e349 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -68,6 +68,9 @@ struct fxregs_state {
/* Default value for fxregs_state.mxcsr: */
#define MXCSR_DEFAULT 0x1f80

+/* Copy both mxcsr & mxcsr_flags with a single u64 memcpy: */
+#define MXCSR_AND_FLAGS_SIZE sizeof(u64)
+
/*
* Software based FPU emulation state. This is arbitrary really,
* it matches the x87 format to make it easier to understand:
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 772a069f8fbf..992a7f8f4988 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -920,6 +920,23 @@ int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
#endif /* ! CONFIG_ARCH_HAS_PKEYS */

/*
+ * Weird legacy quirk: SSE and YMM states store information in the
+ * MXCSR and MXCSR_FLAGS fields of the FP area. That means if the FP
+ * area is marked is unused in the xfeatures header, we need to copy
+ * MXCSR and MXCSR_FLAGS if either SSE or YMM are in use.
+ */
+static inline bool xfeatures_need_mxcsr_copy(u64 xfeatures)
+{
+ if (!(xfeatures & (XFEATURE_MASK_SSE|XFEATURE_MASK_YMM)))
+ return 0;
+
+ if (xfeatures & XFEATURE_MASK_FP)
+ return 0;
+
+ return 1;
+}
+
+/*
* This is similar to user_regset_copyout(), but will not add offset to
* the source data pointer or increment pos, count, kbuf, and ubuf.
*/
@@ -987,6 +1004,13 @@ int copy_xstate_to_kernel(void *kbuf, struct xregs_state *xsave, unsigned int of

}

+ if (xfeatures_need_mxcsr_copy(header.xfeatures)) {
+ offset = offsetof(struct fxregs_state, mxcsr);
+ size = MXCSR_AND_FLAGS_SIZE;
+ __copy_xstate_to_kernel(kbuf, &xsave->i387.mxcsr, offset,
+ size, size_total);
+ }
+
/*
* Fill xsave->i387.sw_reserved value for ptrace frame:
*/
@@ -1069,6 +1093,13 @@ int copy_xstate_to_user(void __user *ubuf, struct xregs_state *xsave, unsigned i

}

+ if (xfeatures_need_mxcsr_copy(header.xfeatures)) {
+ offset = offsetof(struct fxregs_state, mxcsr);
+ size = MXCSR_AND_FLAGS_SIZE;
+ __copy_xstate_to_user(ubuf, &xsave->i387.mxcsr, offset,
+ size, size_total);
+ }
+
/*
* Fill xsave->i387.sw_reserved value for ptrace frame:
*/
@@ -1121,6 +1152,12 @@ int copy_kernel_to_xstate(struct xregs_state *xsave, const void *kbuf)
}
}

+ if (xfeatures_need_mxcsr_copy(xfeatures)) {
+ offset = offsetof(struct fxregs_state, mxcsr);
+ size = MXCSR_AND_FLAGS_SIZE;
+ memcpy(&xsave->i387.mxcsr, kbuf + offset, size);
+ }
+
/*
* The state that came in from userspace was user-state only.
* Mask all the user states out of 'xfeatures':
@@ -1176,6 +1213,13 @@ int copy_user_to_xstate(struct xregs_state *xsave, const void __user *ubuf)
}
}

+ if (xfeatures_need_mxcsr_copy(xfeatures)) {
+ offset = offsetof(struct fxregs_state, mxcsr);
+ size = MXCSR_AND_FLAGS_SIZE;
+ if (__copy_from_user(&xsave->i387.mxcsr, ubuf + offset, size))
+ return -EFAULT;
+ }
+
/*
* The state that came in from userspace was user-state only.
* Mask all the user states out of 'xfeatures':
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