Re: [Xen-devel] [PATCH v4 01/15] re-work commit 3e06b989 "IOMMU: make page table population preemptible"...

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.08.18 at 15:40, <paul.durrant@xxxxxxxxxx> wrote:
> ...to simplify the implementation and turn need_iommu back into a boolean.
> 
> As noted in [1] the tri-state nature of need_iommu after commit 3e06b989 is
> confusing, as is the implementation of pre-emption using relmem_list.
> 
> This patch instead uses a simple count of pages already populated stored in
> the x86 variant of struct arch_iommu and skips over that number of pages
> if arch_iommu_populate_page_table() is re-invoked after pre-emption.

Well, yes, I would have used that model in said commit if it was reliable,
but it isn't: What if the list of pages changed between two (re-)invocations?
Furthermore, with huge enough a guest even the skipping of the already
processed several million pages may exhaust the time acceptable between
preemption points.

Jan



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