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

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Message-ID: <4E5CCC0B.2010900@redhat.com>
Date:	Tue, 30 Aug 2011 14:39:55 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Duncan Sands <baldrick@...e.fr>
CC:	Zachary Amsden <zamsden@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH x86 kvm] Fix register corruption in pvclock_scale_delta

On 08/30/2011 11:58 AM, Duncan Sands wrote:
> 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),

Thanks, applied.

-- 
error compiling committee.c: too many arguments to function

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