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

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Thursday, June 26, 2014 3:30 AM
> All,
> VT-d code currently has a number of cases where completion of certain
> operations is being waited for by way of spinning. The various instances
> can be identified relatively easily by grep-ing for all uses of
> DMAR_OPERATION_TIMEOUT (the majority of instances use that
> variable indirectly through IOMMU_WAIT_OP()), allowing for loops of
> up to 1 second. While in many of the cases this _may_ be acceptable
> (which would need to be proven for each individual case, also taking into
> consideration how many of these spinning loops may be executed in a
> row with no preemption/scheduling in between), the invalidation case
> seems particularly problematic: Using DMAR_OPERATION_TIMEOUT is
> a mistake here in the first place, as the timeout here isn't related to
> response times by the IOMMU engine. Instead - with ATS in use - the
> specification mandates a timeout of 1 _minute_ (with a 50% slack, the
> meaning of which none of us [Andrew and Malcolm brought this issue
> to my attention in private talks on the hackathon] was able to really
> interpret in a sensible way).

yes, that's not a good design. Most waits happen in IOMMU initialization,
where 1s timeout is less a big issue. At runtime cache/tlb flush and root
entry manipulation are definitely not good with long spinning .

> So there are two things that need doing rather sooner than later:
> First and foremost the ATS case needs to be taken into consideration
> when doing invalidations. Obviously we can't spin for a minute, so
> invalidation absolutely needs to be converted to a non-spinning model.
> We realize this isn't going to be trivial, which is why we bring this up
> here rather than coming forward with a patch right away.

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.

> Second, looking at Linux (which interestingly enough also spins, and
> that even without any timeout) there are flags in the fault status
> register that can be used to detect at least some loop abort conditions.
> We should definitely make use of anything that can shorten these
> spinning loops (as was already done in commit dd6d87a4 ["VT-d: drop
> redundant calls to invalidate_sync()"] as a very tiny first step). The
> main problem with trying to clone at least some of the functionality
> from Linux is that I'm not convinced the replaying they do can
> actually do anything good. Plus it's clear that - spinning or not - the
> consequences of an invalidation request not completing successfully
> need to be taken care of (and it's of no help that in all cases I looked
> at so far errors passed up from the leaf functions sooner or later
> get dropped on the floor or mis-interpreted).
> And finally, all other spinning instances need to be audited to make
> sure they can't add up to multiple-second spins (iirc we can't
> tolerate more than about 4s without running into time problems on
> certain hardware).

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? :-)


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