linux-kernel - Re: [PATCH x86 kvm] Fix register corruption in pvclock_scale

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Message-ID: <4E5CA62E.8060304@free.fr>
Date:	Tue, 30 Aug 2011 10:58:22 +0200
From:	Duncan Sands <baldrick@...e.fr>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH x86 kvm] Fix register corruption in pvclock_scale_delta

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@...e.fr>

--- 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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