Re: [Xen-ia64-devel] Pagesize for XEN

[tgingold@xxxxxxx]

Quoting Juergen Gross <juergen.gross@xxxxxxxxxxxxxxxxxxx>:

> Hi,
>
> for our mainframe operating system we need support of 4kB pagesize in XEN.
> I plan to do some patches to achieve this in the next 2 or 3 weeks.
Interesting work!
I think you should be able to configure Xen for 4kB pages.  And it should work
(but TBC).  You should really try this first as you will catch the simplest
problems.

> To avoid any performance degradation for linux or windows guests, my plan is
> to introduce a per-domain or per-vcpu minimal page size. A domain would start
> with the default pagesize (currently 16kB) and in case of usage of a smaller
> page (either via itc, itr or vhpt) the hypervisor should switch to the new
> size by reorganizing the xen-vhpt's of that domain (e.g. doing an implicit
> ptc and setting the new minimal pagesize).
This is a worth optimization but should work without it.

> I'm not sure how to handle grantpages, however. After a quick look into the
> sources I think the pagesize of grantpages is statically defined in the XEN
> configuration (16kB default).
Yes.

> The cleanest solution would be to add the pagesize for grantpages to the
> interfaces of XEN. On the other hand this would require lots of changes in
> domU and dom0 in architecture independent code.
> So for the moment I will stay with a XEN defined pagesize of grantpages.
> Any problem related to different domU and grantpage pagsizes will be specific
> to the domU using a smaller than default pagesize. As there is no such domU
> up to now, this is just another topic which has to be addressed during the
> "XENification" of the (new) domU.
You could try to build a domU linux with a 4KB page size and see how it fails.
There will be many issues with PV drivers!

To sum up: very interesting and not easy work!

Tristan.

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