x86/EPT: drop questionable mfn_valid() from epte_get_entry_emt()

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 777c71d313 ("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: 81fd0d3ca4 ("x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: 4fdd8d7556
master date: 2024-06-13 16:55:22 +0200

xen/arch/x86/mm/p2m-ept.c

@@ -512,12 +512,6 @@ int epte_get_entry_emt(struct domain *d, gfn_t gfn, mfn_t mfn,
         return -1;
     }
 
-    if ( !mfn_valid(mfn) )
-    {
-        *ipat = true;
-        return MTRR_TYPE_UNCACHABLE;
-    }
-
     /*
      * Conditional must be kept in sync with the code in
      * {iomem,ioports}_{permit,deny}_access().