Re: KVM: pvmmu breakage with gcc 4.3.0

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> 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.

It doesn't, but it seems to me that it should force GCC to assume 
everything is live.  It's a big stick to hit the problem with but it 
seems like the right thing semantically.

>   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.

Right, it's not telling GCC that we touch a particular variable, but 
rather that we may have touched any variable.

Regards,

Anthony Liguori