[xen staging-4.18] x86/EPT: avoid marking non-present entries for re-configuring

[patchbot@xxxxxxx]

commit d31385be5c8e8bc5efb6f8848057bd0c69e8274a
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Jun 26 13:40:11 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Jun 26 13:40:11 2024 +0200

    x86/EPT: avoid marking non-present entries for re-configuring
    
    For non-present entries EMT, like most other fields, is meaningless to
    hardware. Make the logic in ept_set_entry() setting the field (and iPAT)
    conditional upon dealing with a present entry, leaving the value at 0
    otherwise. This has two effects for epte_get_entry_emt() which we'll
    want to leverage subsequently:
    1) The call moved here now won't be issued with INVALID_MFN anymore (a
       respective BUG_ON() is being added).
    2) Neither of the other two calls could now be issued with a truncated
       form of INVALID_MFN anymore (as long as there's no bug anywhere
       marking an entry present when that was populated using INVALID_MFN).
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    master commit: 777c71d31325bc55ba1cc3f317d4155fe519ab0b
    master date: 2024-06-13 16:54:17 +0200
---
 xen/arch/x86/mm/p2m-ept.c | 29 ++++++++++++++++++-----------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/mm/p2m-ept.c b/xen/arch/x86/mm/p2m-ept.c
index 1aa6bbc771..641d61b350 100644
--- a/xen/arch/x86/mm/p2m-ept.c
+++ b/xen/arch/x86/mm/p2m-ept.c
@@ -649,6 +649,8 @@ static int cf_check resolve_misconfig(struct p2m_domain 
*p2m, unsigned long gfn)
             if ( e.emt != MTRR_NUM_TYPES )
                 break;
 
+            ASSERT(is_epte_present(&e));
+
             if ( level == 0 )
             {
                 for ( gfn -= i, i = 0; i < EPT_PAGETABLE_ENTRIES; ++i )
@@ -914,17 +916,6 @@ ept_set_entry(struct p2m_domain *p2m, gfn_t gfn_, mfn_t 
mfn,
 
     if ( mfn_valid(mfn) || p2m_allows_invalid_mfn(p2mt) )
     {
-        bool ipat;
-        int emt = epte_get_entry_emt(p2m->domain, _gfn(gfn), mfn,
-                                     i * EPT_TABLE_ORDER, &ipat,
-                                     p2mt);
-
-        if ( emt >= 0 )
-            new_entry.emt = emt;
-        else /* ept_handle_misconfig() will need to take care of this. */
-            new_entry.emt = MTRR_NUM_TYPES;
-
-        new_entry.ipat = ipat;
         new_entry.sp = !!i;
         new_entry.sa_p2mt = p2mt;
         new_entry.access = p2ma;
@@ -940,6 +931,22 @@ ept_set_entry(struct p2m_domain *p2m, gfn_t gfn_, mfn_t 
mfn,
             need_modify_vtd_table = 0;
 
         ept_p2m_type_to_flags(p2m, &new_entry);
+
+        if ( is_epte_present(&new_entry) )
+        {
+            bool ipat;
+            int emt = epte_get_entry_emt(p2m->domain, _gfn(gfn), mfn,
+                                         i * EPT_TABLE_ORDER, &ipat,
+                                         p2mt);
+
+            BUG_ON(mfn_eq(mfn, INVALID_MFN));
+
+            if ( emt >= 0 )
+                new_entry.emt = emt;
+            else /* ept_handle_misconfig() will need to take care of this. */
+                new_entry.emt = MTRR_NUM_TYPES;
+            new_entry.ipat = ipat;
+        }
     }
 
     if ( sve != -1 )
--
generated by git-patchbot for /home/xen/git/xen.git#staging-4.18