Re: KVM: pvmmu breakage with gcc 4.3.0

Avi Kivity
2008-06-26T11:43:15Z

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.