Re: [PATCH] x86/mm: do not mark IO regions as Xen heap

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Sep 10, 2020 at 04:41:41PM +0200, Jan Beulich wrote:
> On 10.09.2020 15:35, Roger Pau Monne wrote:
> > arch_init_memory will treat all the gaps on the physical memory map
> > between RAM regions as MMIO and use share_xen_page_with_guest in order
> > to assign them to dom_io. This has the side effect of setting the Xen
> > heap flag on such pages, and thus is_special_page would then return
> > true which is an issue in epte_get_entry_emt because such pages will
> > be forced to use write-back cache attributes.
> > 
> > Fix this by introducing a new helper to assign the MMIO regions to
> > dom_io without setting the Xen heap flag on the pages, so that
> > is_special_page will return false and the pages won't be forced to use
> > write-back cache attributes.
> > 
> > Fixes: 81fd0d3ca4b2cd ('x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()')
> > Suggested-by: Jan Beulich <jbeulich@suse.com>
> > Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> 
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> albeit I'm inclined to add, while committing, a comment ...
> 
> > --- a/xen/arch/x86/mm.c
> > +++ b/xen/arch/x86/mm.c
> > @@ -271,6 +271,18 @@ static l4_pgentry_t __read_mostly split_l4e;
> >  #define root_pgt_pv_xen_slots ROOT_PAGETABLE_PV_XEN_SLOTS
> >  #endif
> >  
> > +static void __init assign_io_page(struct page_info *page)
> > +{
> > +    set_gpfn_from_mfn(mfn_x(page_to_mfn(page)), INVALID_M2P_ENTRY);
> > +
> > +    /* The incremented type count pins as writable. */
> > +    page->u.inuse.type_info = PGT_writable_page | PGT_validated | 1;
> > +
> > +    page_set_owner(page, dom_io);
> > +
> > +    page->count_info |= PGC_allocated | 1;
> > +}
> 
> ... clarifying its relationship with share_xen_page_with_guest().

Sure, I think the relation is quite vague now, since this is no longer
sharing the page (as it's not added to xenpage_list) but rather
assigning it to dom_io.

In general I feel this is all quite confusing, and would benefit from
having some clear rules about what flags and assigned domains non-RAM
pages have, and how they should be added to the p2m.

Thanks, Roger.