Re: [PATCH] Add memory clobber to hypercalls (v2)

[Avi Kivity <avi@qumranet.com>]

Anthony Liguori wrote:
> Hypercalls can modify arbitrary regions of memory.  Make sure to indicate this
> in the clobber list.  This fixes a hang when using KVM_GUEST kernel built with
> GCC 4.3.0.
>
> This was originally spotted and analyzed by Marcelo.
>
> Since v1, I've also added a "m" constraint for the inputs to the hypercall.
> This was suggested by Christian since it's not entirely clear whether a memory
> clobber will force the data to be in memory before the asm statement.  In the
> very least, it helps to be more conservative.
>
> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
>
> @@ -80,7 +81,9 @@ static inline long kvm_hypercall1(unsigned int nr, unsigned long p1)
>  	long ret;
>  	asm volatile(KVM_HYPERCALL
>  		     : "=a"(ret)
> -		     : "a"(nr), "b"(p1));
> +		     : "a"(nr), "b"(p1),
> +		       "m"(*(char *)p1)
> +		     : "memory");
>  	return ret;
>  }
>  
>   

Those are physical addresses, not virtual, and on i386 the addresses are 
split across multiple registers.

However a small test program shows that the memory clobber does work 
with gcc 4.3, so I'll pick the earlier patch.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.