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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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
> path
> > 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
> ops.]
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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