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

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Sent: 10 September 2020 11:35
> To: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper 
> <andrew.cooper3@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>;
> Paul Durrant <paul@xxxxxxx>
> Subject: Re: [PATCH] x86/hvm: don't treat MMIO pages as special ones 
> regarding cache attributes
> 
> 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.
> 

In general they are not marked Xen heap then they will be considered domheap 
and will go through the normal free-ing path on domain destruction. Of course 
this doesn't apply for a system domain that never gets destroyed.

  Paul