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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/05/2015 09:52, 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
> think "serializing" needs to be changed to "fencing", as serialization
> is not what we really care about here. If you and the maintainers
> agree, I could certainly fix up both aspects while committing.

On the subject of writebacks, we should get around to alternating-up the
use of clflushopt and clwb, either of which would be better than a
clflush in this case (avoiding the need for the leading mfence).

However, the ISA extension document does not indicate which processors
will have support for these new instructions.

~Andrew

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