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

On 27 Jul 2006, at 18:31, Ian Pratt wrote:

The obvious thing to do is emulate the first 4 updates to a particular
page, and only then switch to batched mode. Slows down the batched
a bit, but stops it firing in many cases where it is no help.

Why? There should be no overhead to just building batches on the stack
(or a per vcpu area) and flushing at the end of the page. Certainly if
we were to keep wrpt it would make sense to take a few emulations faults
first on a page before engaging wrpt, but for explicit batches we don't
need any smarts.

[Although the batching strategy would (currently) work for Linux, we do
have to bare in mind that some OSes (possibly NetBSD) won't rely on a
lock to protect updates to pagetables and will use individual atomic

It wasn't clear to me there was a batching strategy that would integrate nicely with Linux generic mm code and be useful to any other OSes. We don't particularly want to accumulate OS-specific hacks unless it's a significant win (which we have no evidence it would be here).

 -- Keir

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