Re: [Xen-devel] [PATCH v2 06/13] iommu: Add extra use_iommu argument to iommu_domain_init()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.07.17 at 19:26, <olekstysh@xxxxxxxxx> wrote:
> The presence of this flag lets us know that the guest domain has statically
> assigned devices which will most likely be used for passthrough
> and as the result the IOMMU is expected to be used for this domain.
> 
> Taking into the account this hint when dealing with non-shared IOMMUs
> we can populate IOMMU page tables before hand avoid going through
> the list of pages at the first assigned device.
> As this flag doesn't cover hotplug case, we will continue to populate
> IOMMU page tables on the fly.

While of course it would have been nice if I would have found time
earlier to look at this patch (and hence closer to when the discussion
happened), I still don't see it being made sufficiently clear here why
current behavior (without a need for such a flag) is a problem for the
non-shared IOMMU case on ARM, when it isn't on x86.

The patch itself looks mechanical enough that it could get my ack,
but I really want to understand the background without having to
dig out old discussions (which would be even more difficult for
future archaeologists running into this change in a few years time).

Jan


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