Re: [Xen-devel] VT-d spin loops

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.07.14 at 10:00, <yang.z.zhang@xxxxxxxxx> wrote:
> Andrew Cooper wrote on 2014-07-10:
>> On 10/07/14 00:22, Tian, Kevin wrote:
>>> ATS should be fine. Device TLB can ONLY be validated through qinval
>>> interface, which is asynchronous so no need to consider 1 minute
>>> timeout even in current spinning model.
>> 
>> There are currently no asynchronous invalidations in Xen. ATS
>> certainly is a problem.
> 
> How Linux upstream handle ATS? Does it have any asynchronous invalidations 
> mechanism?

Not according to my inspection of the code.

>>> In general yes a non-spinning model is better, but it requires
>>> non-trivial change to make all IOMMU operations asynchronous. If ATS
>>> is not a concern, is it still worthy of change besides auditing existing 
> usages?
>> 
>> Even if the invalidation is only at the IOMMU, waiting milliseconds
>> for the completion is still time better spent elsewhere, such as running 
> VMs.
>> 
>> Do you have any numbers for typical completion times for invalidate 
> requests?
>> 
> 
> The invalidations are completed fairly quickly by hardware. So the cost for 
> spin can be ignored?

No, we have to be prepared for a timeout to occur, without killing
the entire host (killing the guest owning affected device would be
acceptable as a consequence), even more so with the longer
timeouts mandated by ATS.

Jan


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