[PATCH v2 for-4.19 3/3] x86/EPT: drop questionable mfn_valid() from epte_get_entry_emt()

[Jan Beulich <jbeulich@xxxxxxxx>]

mfn_valid() is RAM-focused; it will often return false for MMIO. Yet
access to actual MMIO space should not generally be restricted to UC
only; especially video frame buffer accesses are unduly affected by such
a restriction.

Since, as of ???????????? ("x86/EPT: avoid marking non-present entries
for re-configuring"), the function won't be called with INVALID_MFN or,
worse, truncated forms thereof anymore, we call fully drop that check.

Fixes: 81fd0d3ca4b2 ("x86/hvm: simplify 'mmio_direct' check in 
epte_get_entry_emt()")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
Considering that we've just declared PVH Dom0 "supported", this may well
qualify for 4.19. The issue was specifically very noticeable there.
---
v2: Different approach (and hence different title and description).

--- a/xen/arch/x86/mm/p2m-ept.c
+++ b/xen/arch/x86/mm/p2m-ept.c
@@ -501,12 +501,6 @@ int epte_get_entry_emt(struct domain *d,
         return -1;
     }
 
-    if ( !mfn_valid(mfn) )
-    {
-        *ipat = true;
-        return X86_MT_UC;
-    }
-
     /*
      * Conditional must be kept in sync with the code in
      * {iomem,ioports}_{permit,deny}_access().