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Subject[PATCH 4.16 062/110] x86/nospec: Simplify alternative_msr_write()
Date
4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds@linux-foundation.org>

commit 1aa7a5735a41418d8e01fa7c9565eb2657e2ea3f upstream

The macro is not type safe and I did look for why that "g" constraint for
the asm doesn't work: it's because the asm is more fundamentally wrong.

It does

movl %[val], %%eax

but "val" isn't a 32-bit value, so then gcc will pass it in a register,
and generate code like

movl %rsi, %eax

and gas will complain about a nonsensical 'mov' instruction (it's moving a
64-bit register to a 32-bit one).

Passing it through memory will just hide the real bug - gcc still thinks
the memory location is 64-bit, but the "movl" will only load the first 32
bits and it all happens to work because x86 is little-endian.

Convert it to a type safe inline function with a little trick which hands
the feature into the ALTERNATIVE macro.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/include/asm/nospec-branch.h | 19 ++++++++++---------
1 file changed, 10 insertions(+), 9 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -241,15 +241,16 @@ static inline void vmexit_fill_RSB(void)
#endif
}

-#define alternative_msr_write(_msr, _val, _feature) \
- asm volatile(ALTERNATIVE("", \
- "movl %[msr], %%ecx\n\t" \
- "movl %[val], %%eax\n\t" \
- "movl $0, %%edx\n\t" \
- "wrmsr", \
- _feature) \
- : : [msr] "i" (_msr), [val] "i" (_val) \
- : "eax", "ecx", "edx", "memory")
+static __always_inline
+void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
+{
+ asm volatile(ALTERNATIVE("", "wrmsr", %c[feature])
+ : : "c" (msr),
+ "a" (val),
+ "d" (val >> 32),
+ [feature] "i" (feature)
+ : "memory");
+}

static inline void indirect_branch_prediction_barrier(void)
{

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