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

Keir Fraser wrote:

On 31 Jul 2006, at 10:32, Zachary Amsden wrote:

It would allow set_pte() to switch between explicit queuing and 'direct' writing. We moved away from the former a few years back as doing it everywhere made a mess of the generic Linux mm code and it was hard to reason whether our patches were correct. I guess doing it for the most important subset of mm routines is not so bad. It's a shame that, although many set_pte() call sites could determine statically whether or not they will batch, we'd end up with a dynamic run-time test everywhere (unless I'm mistaken) -- I wonder if that has a measurable cost?

We've actually seen a benefit for this, despite the cost of the non-static parameters, for both VMI Linux with shadow pagetables on ESX and VMI Linux with direct pagetables on Xen. Turns out that as long as the call EIP is predictable, the parameters do not necessarily need to be so, and modern processors are getting much better at branch prediction.

You mean that the benefit of batching outweighs the cost of an extra test-and-branch in the middle of a loop, or that the extra test-and-branch simply has unmeasurable overhead? The former is to be expected, but I'd be worried about other call sites where batching does not happen, and an effect on those.

The extra test-and-branch has unmeasurable overhead. In the implementation we had chosen, there was already a branch requirement on the set_pte call anyway, to potentially delay the pte update so that it can piggyback onto a page invalidation with just one hypercall. Combining the two branches into one is trivial, and the cost of one extra branch here seems to be invisible. We were getting better numbers for MMU related workloads with VMI-Linux than XenoLinux was. I don't have hard numbers on this, and even if I did, it would take some time to get them approved for public distribution. For that I must apologize. But avoiding the changes that would otherwise be required - a full set of pte and tlb functions which could be delayed, as well as combining the pte update and invlpg into a single call - seemed worth a single branch. I'm not even convinced these changes can be done in a way that would be safe for all architectures. Of course, I may be wrong on that point - but there is no simple way I see to do it that affords the strong reasoning about correctness that the enter / leave semantic does.

Doing explicit batching exactly where it counts, under protection of locks, so that SMP safety is guaranteed turns out to be really easy, as well as a nice win.

If the run-time check cost really isn't an issue (I'd like to see numbers), we'd likely use this new interface in preference to implicitly batched writable pagetables and would support its inclusion in the kernel.

Sorry about not having numbers. My biggest question is - do you need any other information than simply a single state variable to use explicit batching? I thought, and Jeremy and Chris both pointed out as well, that Xen could potentially use the information about which PT to unhook to take advantage of writable pagetables. But, if that is not the direction you are going, then it seems this information is not so relevant for the explicit batching. 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.


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