IOMMU page table freeing

[Jan Beulich <jbeulich@xxxxxxxx>]

Hello,

while I don't expect this case to occur often in practice, for
superpage support we will need to be able to correctly free a
page table (hierarchy) after replacing its mapping range by a
superpage. Following P2M by carrying out an immediate iotlb flush
prior to synchronously freeing the memory looks, to me at least,
out of question as an option - the latest when considering ATS
the flush may simply take too long.

Making use of RCU doesn't look like a good option either, as
this would require callers of map/unmap + flush to enclose the
whole group of operations in an RCU-read-locked region. Yet I
think we want to avoid to concern callers with details of the
implementation of the IOMMU operations.

Which I think leaves deferring the freeing to a softirq or
tasklet, of which the latter - to me - would seem the better
(easier) choice. If you have any alternative / better suggestions
I'd appreciate a reply; ideally you would also reply if you
simply agree.

FAOD I don't think I want to make an attempt just yet to care
about the case of flushes getting carried out asynchronously:
That would require a means to signal to the freeing function
which prior page table pages are ready to be freed. For now I'm
rather considering to merely accumulate these pages simply on a
(perhaps per-CPU) list, for the tasklet handler to consume.

Thanks, Jan