Re: KVM: pvmmu breakage with gcc 4.3.0

[Anthony Liguori <aliguori@us.ibm.com>]

Christian Borntraeger wrote:
> Am Donnerstag, 26. Juni 2008 schrieb Avi Kivity:
>   
>> I don't think "p" should force the contents into memory?  Perhaps 
>> "m"(*(char *)buffer)?
>>
>> Anthony, I don't see why a memory clobber would tell gcc that the 
>> variables is actually used.  The problem is with the void * -> unsigned 
>> long cast (__pa), once that happens gcc loses track.  It's probably 
>> needed anyway since hypercalls _do_ clobber memory.
>>     
>
> I I think about that again, the correct solution should be to use 2 input 
> constraints for parameters together with the memory clobber on hypercall.
>
> I think something like the following covers all cases:
>
> static inline long kvm_hypercall1(unsigned int nr, unsigned long p1)
> {
> 	long ret;
> 	asm volatile(KVM_HYPERCALL
> 		     : "=a"(ret)
> 		     : "a"(nr), "b"(p1), "m"(*(char *) p1)
> 		     : "memory" );
> 	return ret;
> }
>
>
> The address and the memory content of p1 (if it is a pointer) is not ommitted 
> by gcc. Furthermore, the memory clobbering nature of a hypercall is specified 
> as well.
>   

Isn't that redundant though?  Shouldn't a memory clobber force GCC to 
assume that anything that's reachable is live?

Regards,

Anthony Liguori

> Christian
>