Re: [Xen-devel] [PATCH 1/3] xen/vt-d: need barriers to workaround CLFLUSH

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.05.15 at 12:39, <tiejun.chen@xxxxxxxxx> wrote:
> On 2015/5/4 16:52, Jan Beulich wrote:
>>>>> On 04.05.15 at 04:16, <tiejun.chen@xxxxxxxxx> wrote:
>>> --- a/xen/drivers/passthrough/vtd/x86/vtd.c
>>> +++ b/xen/drivers/passthrough/vtd/x86/vtd.c
>>> @@ -56,7 +56,9 @@ unsigned int get_cache_line_size(void)
>>>
>>>   void cacheline_flush(char * addr)
>>>   {
>>> +    mb();
>>>       clflush(addr);
>>> +    mb();
>>>   }
>>
>> I think the purpose of the flush is to force write back, not to evict
>> the cache line, and if so wmb() would appear to be sufficient. As
>> the SDM says that's not the case, a comment explaining why wmb()
>> is not sufficient would seem necessary. Plus in the description I
> 
> Seems wmb() is not sufficient here.
> 
> "CLFLUSH is only ordered by the MFENCE instruction. It is not guaranteed 
> to be ordered by any other fencing, serializing or other CLFLUSH 
> instruction."

Right - that's what I said in the second sentence.

Jan


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