Re: porting xen-detect ASM code into a shared library

[Guillaume Rousse <Guillaume.Rousse@inria.fr>]

Keir Fraser a écrit :
> 
> 
> On 10/7/08 10:21, "Guillaume Rousse" <Guillaume.Rousse@inria.fr> wrote:
> 
>> I'm trying to port xen-detect.c ASM code into cfengine, which is
>> currently build as a shared library. However, I'm facing a compilation
>> error:
>> misc.c: In function 'Xen_cpuid':
>> misc.c:1376: error: can't find a register in class 'BREG' while
>> reloading 'asm'
>> misc.c:1376: error: 'asm' operand has impossible constraints
>>
>>  From 
> 
> See tools/libxc/xc_cpuid_x86.c for an example of how to work around this.
> Essentially you turn the "=b" constraint into "=r". You then push/pop ebx
> (to save/restore it) and you 'mov %%ebx,%1' to save away ebx's value before
> restoring its original value.
Thanks, I see the idea, but I feel like a typing monkey here because of 
the unknown syntax. Is this supposed to be OK ?

void Xen_cpuid(uint32_t idx, uint32_t *eax, uint32_t *ebx, uint32_t *ecx,
                   uint32_t *edx)
{
     asm volatile (
         "test %1,%1 ; jz 1f ; ud2a ; .ascii \"xen\" ;"

#ifdef __i386__
          "push %%ebx; cpuid; mov %%ebx,%1; pop %%ebx"
#else
          "push %%rbx; cpuid; mov %%ebx,%1; pop %%rbx"
#endif

         : "=a" (*eax), "=r" (*ebx), "=c" (*ecx), "=d" (*edx)
         : "0" (idx), "1" (0) );
}

Also, the final line was originally:
: "0" (idx), "1" (pv_context) );

I turned it into the current way because I don't need the check in pv 
context, but I guess it might be simplified.
-- 
Guillaume Rousse
Moyens Informatiques - INRIA Futurs
Tel: 01 69 35 69 62