[Xen-devel] [RFC] x86: cacheability page attributes

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

Attached draft patch is supposed to help dealing with tracking cacheability
attributes on x86, specifically to prevent page aliases using different
cacheability attributes. The problem with the approach of extending the
set of page types (by splitting PGT_writable_page) is that a simple update
of a pte which changes nothing but _PAGE_PWT or _PAGE_PCD doesn't
work, as the scheme requires the old type (e.g. simple writeable page) to
first drop its type reference count to zero, before the new type can be
established.

While in all previous scenarios this doesn't present a problem, here it simply
seems wrong: If the only use is through the pte being updated, this one
reference shouldn't be considered. As I understand it, racing requests to
bump the type reference count could be avoided by clearing PGT_validated
while retaining the single reference, but what I don't have a reasonable
idea for is how (and where) to handle the transition while retaining the
type reference.

Thanks, Jan

Attachment: page-cacheability.patch
Description: Text document

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