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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Sep 21, 2020 at 04:22:26PM +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()')
> 
> Is this really the correct commit? I had this queued for backport,
> and ended up discarding it from the queue based on this tag, just
> to then noticing that I remembered correctly that I did backport
> ca24b2ffdbd9 ("x86/hvm: set 'ipat' in EPT for special pages") to
> the stable trees already. Isn't it _this_ commit which the change
> here fixes?

My bisection pointed to that exact commit as the one that broke my PVH
dom0 setup, so yes, I'm quite sure that's the commit at least on the
box that I've bisected it.

ca24b2ffdbd9 was still fine because previous to the is_special_page
check loop there was still the `if ( direct_mmio ) ...` handling,
which made all MMIO regions except for the APIC access page forcefully
have it's cache attributes set to UC.

Roger.