On Mon, Feb 22, 2010 at 04:57:29PM +0200, Avi Kivity wrote:
> On 02/22/2010 04:45 PM, Marcelo Tosatti wrote:
>> On Mon, Feb 22, 2010 at 04:23:32PM +0200, Avi Kivity wrote:
>>
>>> On 02/22/2010 03:59 PM, Marcelo Tosatti wrote:
>>>
>>>> Cc: "Michael S. Tsirkin"<mst@xxxxxxxxxx>
>>>> Signed-off-by: Marcelo Tosatti<mtosatti@xxxxxxxxxx>
>>>>
>>>> Index: qemu/kvm-all.c
>>>> ===================================================================
>>>> --- qemu.orig/kvm-all.c
>>>> +++ qemu/kvm-all.c
>>>> @@ -718,6 +718,9 @@ static int kvm_handle_io(uint16_t port,
>>>> return 1;
>>>> }
>>>>
>>>> +/* FIXME: arch dependant, x86 version */
>>>> +#define smp_wmb() asm volatile("" ::: "memory")
>>>> +
>>>>
>>> sfence?
>>>
>> There is no need (for this case). Older read cannot be reordered with
>> write, writes are not reordered with other writes, writes by a single
>> processor are observed in the same order by all processors.
>>
>
> Well, Linux does use sfence.
At least on 64 bit it doesnt.
> Perhaps it's only needed for WC writes
> (movnti and friends), but better be careful here.
>
>>> what about other arches?
>>>
>> They need to be fixed? PPC needs an instruction apparently.
>>
>> Is there any objection to including this patch?
>>
>
> I imagine all arches need an instruction. For reads as well.
>
> Note, gcc has a __sync_synchronize() builtin that compiles to mfence on
> x86. We might use that as a baseline for both rmb and wmb, and let each
> arch override it incrementally.
This it what my patch did. Note it only works well for recent gcc.
> --
> error compiling committee.c: too many arguments to function
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