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Re: [Xen-devel] [PATCH] grant table and bogus mfns

On Tue, 2007-11-13 at 16:45 +0000, Keir Fraser wrote:
> As for GNTMAP_nocache, I think actually we should not trust the mapper to
> get the cache attributes correct (because sometimes if the attributes are
> wrong the system can be destabilised).

Is this just (or mainly?) a concern for RAM pages?  If not, you'll have
the same problem when using the domctl iomem_permission and
ioremap_nocache() in the guest.

>  Actually we now in xen-unstable have
> cache-attribute tracking for RAM pages. The cache attributes for the granted
> mapping should pick up PAT/PWT/PCD from the page's count_info field. If dom0
> has the page mapped with the right attributes this will then do the right
> thing immediately. If dom0 doesn't have the page mapped then we'll need to
> do a bit more work...

dom0 does have the page mapped, but I think there is no page_info struct
(and so no count_info) for some I/O memory pages.  Perhaps a short-term
fix would be to remove the GNTMAP_nocache option that I've added and
instead just make all I/O memory pages that get mapped through this
mechanism forced to _PAGE_PCD.  That way the mapper doesn't get to
choose (and we loose the ability to do an ioremap() equivalent as
opposed to an ioremap_nocache() equivalent, but that's not a big problem
right now).  

If that sounds agreeable I can spin another patch.


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