On Thu, Sep 10, 2020 at 11:27:49AM +0200, Jan Beulich wrote: > On 09.09.2020 16:50, Roger Pau Monne wrote: > > MMIO regions below the maximum address on the memory map can have a > > backing page struct that's shared with dom_io (see x86 > > arch_init_memory and it's usage of share_xen_page_with_guest), and > > thus also fulfill the is_special_page check because the page has the > > Xen heap bit set. > > > > This is incorrect for MMIO regions when is_special_page is used by > > epte_get_entry_emt, as it will force direct MMIO regions mapped into > > the guest p2m to have the cache attributes set to write-back. > > > > Add an extra check in epte_get_entry_emt in order to detect pages > > shared with dom_io (ie: MMIO regions) and don't force them to > > write-back cache type on that case. > > Did you consider the alternative of not marking those pages as Xen > heap ones? In particular when looking at it from this angle I > consider it at least odd for non-RAM (or more precisely non-heap) > pages to get marked this way.
I wasn't sure whether this could cause issues in other places of the code that would rely on this fact and such change seemed more risky IMO. > And I can't currently see anything > requiring them to be marked as such - them being owned by DomIO is > all that's needed as it seems. Should those pages then simply be assigned to dom_io and set the appropriate flags (PGC_allocated | 1), or should share_xen_page_with_guest be modified to not set the PGC_xen_heap flag? I see that such addition was done in a2b4b8c2041, but I'm afraid I don't fully understand why share_xen_page_with_guest needs to mark pages as Xen heap. Thanks, Roger.
