Re: KVM: pvmmu breakage with gcc 4.3.0

[Anthony Liguori <anthony@codemonkey.ws>]

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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.
>   

I think a better fix is to add a "memory " clobber to the hypercalls.  
This isn't really a GCC bug since it doesn't realize that hypercalls can 
touch memory.

See the attached patch.

Avi: please push this for 2.6.26 if possible.

Regards,

Anthony Liguori

> 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));
> +
>  	do {
>  		a1 = __pa(buffer);
>  		a2 = 0;   /* on i386 __pa() always returns <4G */
> --
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>   


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