Re: [PATCH] Add hypercall to mark superpages to improve performance

[Keir Fraser <keir.fraser@eu.citrix.com>]

On 02/05/2010 14:34, "Dave McCracken" <dcm@mccr.org> wrote:

> One outstanding issue I see is how to handle readonly mappings.  If we follow
> the model of regular page typecount, readonly mappings of superpages would not
> conflict with the "conflicts with superpage" type.  This means a subsequent
> attempt to change it to a read/write mapping could fail, just like with a
> regular page.  Or we could count all mappings of superpages as if they were
> read/write.

I'd keep an extra refcount in superpage_info to track read-only mappings (or
all superpage mappings, as page->count_info does for 4kB mappings). It's
trivial extra space and avoids having unexpected extra restrictions on
read-only superpage mappings.

> What are your thoughts?  It seems fairly simple and elegant to me, and at this
> point I don't see any big holes in it.

It does mean that creating/destroying pagetable pages causes an extra locked
read-modify-write cycle on a non-local cacheline (superpage_info refcount).
Would this be significant? Not sure. I guess we'd only be doing it for
guests with the superpage capability configured, and we could do some
performance comparisons with the capability enabled/disabled. I think
overall I quite like your suggestion.

 -- Keir