Re: [Xen-devel] [PATCH] x86: fix wait code asm() constraints

[Keir Fraser <keir@xxxxxxx>]

On 03/08/2012 09:40, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

> In __prepare_to_wait(), properly mark early clobbered registers. By
> doing so, we at once eliminate the need to save/restore rCX and rDI.
> 
> In check_wakeup_from_wait(), make the current constraints match by
> removing the code that actuall alters registers. By adjusting the
> resume address in __prepare_to_wait(), we can simply re-use the copying
> operation there (rather than doing a second pointless copy in the
> opposite direction after branching to the resume point), which at once
> eliminates the need for re-loading rCX and rDI inside the asm().

First of all, this is code improvement, rather than a bug fix, right? The
asm constraints are correct for the code as it is, I believe.

It also seems the patch splits into two independent parts:

 A. Not sure whether the trade-off of the rCX/rDI save/restore versus more
complex asm constraints makes sense.

 B. Separately, the adjustment of the restore return address, and avoiding
needing to reload rCX/rDI after label 1, as well as avoiding the copy in
check_wakeup_from_wait(), is very nice.

I'm inclined to take the second part only, and make it clearer in the
changeset comment that it is not a bug fix.

What do you think?

 -- Keir

> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/common/wait.c
> +++ b/xen/common/wait.c
> @@ -126,6 +126,7 @@ static void __prepare_to_wait(struct wai
>  {
>      char *cpu_info = (char *)get_cpu_info();
>      struct vcpu *curr = current;
> +    unsigned long dummy;
>  
>      ASSERT(wqv->esp == 0);
>  
> @@ -140,27 +141,27 @@ static void __prepare_to_wait(struct wai
>  
>      asm volatile (
>  #ifdef CONFIG_X86_64
> -        "push %%rax; push %%rbx; push %%rcx; push %%rdx; push %%rdi; "
> +        "push %%rax; push %%rbx; push %%rdx; "
>          "push %%rbp; push %%r8; push %%r9; push %%r10; push %%r11; "
>          "push %%r12; push %%r13; push %%r14; push %%r15; call 1f; "
> -        "1: mov 80(%%rsp),%%rdi; mov 96(%%rsp),%%rcx; mov %%rsp,%%rsi; "
> +        "1: mov %%rsp,%%rsi; addq $2f-1b,(%%rsp); "
>          "sub %%rsi,%%rcx; cmp %3,%%rcx; jbe 2f; "
>          "xor %%esi,%%esi; jmp 3f; "
>          "2: rep movsb; mov %%rsp,%%rsi; 3: pop %%rax; "
>          "pop %%r15; pop %%r14; pop %%r13; pop %%r12; "
>          "pop %%r11; pop %%r10; pop %%r9; pop %%r8; "
> -        "pop %%rbp; pop %%rdi; pop %%rdx; pop %%rcx; pop %%rbx; pop %%rax"
> +        "pop %%rbp; pop %%rdx; pop %%rbx; pop %%rax"
>  #else
> -        "push %%eax; push %%ebx; push %%ecx; push %%edx; push %%edi; "
> +        "push %%eax; push %%ebx; push %%edx; "
>          "push %%ebp; call 1f; "
> -        "1: mov 8(%%esp),%%edi; mov 16(%%esp),%%ecx; mov %%esp,%%esi; "
> +        "1: mov %%esp,%%esi; addl $2f-1b,(%%esp); "
>          "sub %%esi,%%ecx; cmp %3,%%ecx; jbe 2f; "
>          "xor %%esi,%%esi; jmp 3f; "
>          "2: rep movsb; mov %%esp,%%esi; 3: pop %%eax; "
> -        "pop %%ebp; pop %%edi; pop %%edx; pop %%ecx; pop %%ebx; pop %%eax"
> +        "pop %%ebp; pop %%edx; pop %%ebx; pop %%eax"
>  #endif
> -        : "=S" (wqv->esp)
> -        : "c" (cpu_info), "D" (wqv->stack), "i" (PAGE_SIZE)
> +        : "=&S" (wqv->esp), "=&c" (dummy), "=&D" (dummy)
> +        : "i" (PAGE_SIZE), "1" (cpu_info), "2" (wqv->stack)
>          : "memory" );
>  
>      if ( unlikely(wqv->esp == 0) )
> @@ -200,7 +201,7 @@ void check_wakeup_from_wait(void)
>      }
>  
>      asm volatile (
> -        "mov %1,%%"__OP"sp; rep movsb; jmp *(%%"__OP"sp)"
> +        "mov %1,%%"__OP"sp; jmp *(%0)"
>          : : "S" (wqv->stack), "D" (wqv->esp),
>          "c" ((char *)get_cpu_info() - (char *)wqv->esp)
>          : "memory" );
> 
> 
> 
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