Re: [Xen-devel] [PATCH] x86/time: fix scale_delta() inline assembly

[Keir Fraser <keir@xxxxxxx>]

On 26/11/2012 15:23, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

> The way it was coded, it clobbered %rdx without telling the compiler.
> This generally didn't cause any problems except when there are two back
> to back invocations (as in plt_overflow()), as in that case the
> compiler may validly assume that it can re-use for the second instance
> the value loaded into %rdx before the first one.
> 
> Once at it, also properly relax the second operand of "mul" (there's no
> need for it to be in %rdx, or a register at all), and switch away from
> using explicit register names in the instruction operands.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Ugh. Thank you very much, Jan! What a horrible bug.

Acked-by: Keir Fraser <keir@xxxxxxx>

> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -127,8 +127,9 @@ static inline u64 scale_delta(u64 delta,
>          delta <<= scale->shift;
>  
>      asm (
> -        "mul %%rdx ; shrd $32,%%rdx,%%rax"
> -        : "=a" (product) : "0" (delta), "d" ((u64)scale->mul_frac) );
> +        "mul %2 ; shrd $32,%1,%0"
> +        : "=a" (product), "=d" (delta)
> +        : "rm" (delta), "0" ((u64)scale->mul_frac) );
>  
>      return product;
>  }
> 
> 
> 
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