Re: KVM: pvmmu breakage with gcc 4.3.0

[Avi Kivity <avi@qumranet.com>]

Marcelo Tosatti wrote:
> Some pvmmu functions store their commands on stack, and newer GCC
> versions conclude that these commands are unused.
>
> So stick an inline asm statement to convince the compiler otherwise.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 8b7a3cf..c892752 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -55,6 +55,12 @@ static void kvm_mmu_op(void *buffer, unsigned len)
>  	int r;
>  	unsigned long a1, a2;
>  
> +	/*
> +   	 * GCC 4.3.0 concludes that on-stack kvm_mmu_op* is unused and
> +   	 * optimizes its initialization away.
> + 	 */
> +        asm ("" : : "p" (buffer));
> +
>   

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.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.