Re: [PATCH x86 kvm] Fix register corruption in pvclock_scale_delta

[Duncan Sands]

Hi Zachary, this patch can cause a compile failure if the compiler chooses to
use a memory location for mul_frac rather than a register (as allowed by the
"m" in the constraints):

  pvclock.s:229: Error: no instruction mnemonic suffix given and no register 
operands; can't size instruction

In this example the assembler is:

        #APP
        mul -48(%rbp) ; shrd $32, %rdx, %rax
        #NO_APP

A simple solution is to use mulq, as in the following patch:

Signed-off-by: Duncan Sands <baldrick@xxxxxxx>

--- a/arch/x86/include/asm/pvclock.h
+++ b/arch/x86/include/asm/pvclock.h
@@ -44,7 +44,7 @@ static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, 
int shift)
                : "a" ((u32)delta), "1" ((u32)(delta >> 32)), "2" (mul_frac) );
 #elif defined(__x86_64__)
        __asm__ (
-               "mul %[mul_frac] ; shrd $32, %[hi], %[lo]"
+               "mulq %[mul_frac] ; shrd $32, %[hi], %[lo]"
                : [lo]"=a"(product),
                  [hi]"=d"(tmp)
                : "0"(delta),
--
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