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Re: [Xen-devel] [PATCH] turn off writable page tables

Keir Fraser wrote:
If the guest gives explicit hints, and the extra branch on set_pte does not hurt, then I think it makes sense to do straightforward explicit batching. Providing a PT page hint sounds like it could be ambiguous in some contexts too (e.g., the fork loop modifies two PT pages at a time).

Yes, that is why I shyed away from passing PT hints - then you need to deal with the double hint case, and I think unhooking and revalidating two page tables probably does not make sense based on the UP writeable page table numbers.

The explicit batching does have one disadvantage without writable page tables, which is a potential long term maintenance / correctness issue - you must remove read hazards from these encapsulated paths. That is not so hard to do, and not a large general problem, because the batching is explicit rather than implicit, so you can pick paths to batch that are small, compact, and easy to reason about. But nevertheless, a point I would like to make sure you are comfortable with before we all decide these hooks will work for everyone.

Yes, that's why we moved away from this approach before. But previously we did it for *all* pagetable updates, which was a pain. Doing it just for a few important cases, and having the hooks maintained in upstream Linux, makes this rather less of a headache.

Cool.  It sounds like the lazy mode hooks are exactly what you want then?



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