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

>>> On 05.05.15 at 18:11, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 05/05/2015 11:58 AM, Jan Beulich wrote:
>>>>> On 05.05.15 at 17:46, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 05/04/2015 05:14 AM, Andrew Cooper wrote:
>>>> 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.
>>> https://software.intel.com/sites/default/files/managed/0d/53/319433-022.pdf 
>>> We really should add support for this. On shutting down a very large
>>> guest (hundreds of GB) we observed *minutes* spent in flushing IOMMU.
>>> This was due to serializing nature of CLFLUSH.
>> But flushing the IOMMU isn't being done via CPU instructions, but
>> rather via commands sent to the IOMMU. I.e. I'm somewhat
>> confused by your reply.
> I didn't mean flushing IOMMU itself, sorry. I meant 
> __iommu_flush_cache() (or whatever it's equivalent we had in the 
> product, which was 4.1-based).

In that case I wonder how much of that flushing is really necessary
during IOMMU teardown. VT-d maintainers?


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