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

[Zachary Amsden <zach@xxxxxxxxxx>]

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?

Cheers,

Zach

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