Re: [PATCH] x86/hvm: don't treat MMIO pages as special ones regarding cache attributes

[Jan Beulich <jbeulich@xxxxxxxx>]

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. 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.

Jan