Re: [PATCH] turn off writable page tables

[Zachary Amsden <zach@vmware.com>]

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