Re: [PATCH] Revert "x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()"

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.09.2020 19:09, Roger Pau Monne wrote:
> This reverts commit 81fd0d3ca4b2cd309403c6e8da662c325dd35750.
> 
> Original commit only takes into account the APIC access page being a
> 'special' page, but when running a PVH dom0 there are other pages that
> also fulfill the 'special' page check but shouldn't have it's cache
> type set to WB.
> 
> For example the ACPI regions are identity mapped into the guest but
> are also Xen heap pages, and forcing those to WB cache type is wrong.

Why is this wrong? Xen heap pages are RAM ones. Quite the opposite,
_not_ forcing them to WB is going to cause cachability conflicts,
unless their direct mapping entries also got changed to UC (which I
think we want to avoid if at all possible). The cachability we
select here is the one to be used by the hardware, not the one
perceived by a domain (be it Dom0 or a DomU).

> I've discovered this while trying to boot a PVH dom0, which fail to
> boot with this commit applied.

And what exactly is the deeper reason of the failure?

Jan