[Xen-changelog] [xen stable-4.4] x86/HVM: fix memory type merging in epte_get_entry_emt()

[patchbot@xxxxxxx]

commit 6fc747461edc2dc920d07a247acda9552c7bbfb9
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Mar 14 17:27:42 2014 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Mar 14 17:27:42 2014 +0100

    x86/HVM: fix memory type merging in epte_get_entry_emt()
    
    Using the minimum numeric value of guest and host specified memory
    types is too simplistic - it works only correctly for a subset of
    types. It is in particular the WT/WP combination that needs conversion
    to UC if the two types conflict.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>
    Acked-by: Keir Fraser <keir@xxxxxxx>
    master commit: b99113b9d5fac5149de8496f55afa00e285b1ff3
    master date: 2014-03-10 11:03:53 +0100
---
 xen/arch/x86/hvm/mtrr.c |   32 +++++++++++++++++++++++++++++++-
 1 files changed, 31 insertions(+), 1 deletions(-)

diff --git a/xen/arch/x86/hvm/mtrr.c b/xen/arch/x86/hvm/mtrr.c
index e7c0fd9..b33bf34 100644
--- a/xen/arch/x86/hvm/mtrr.c
+++ b/xen/arch/x86/hvm/mtrr.c
@@ -719,5 +719,35 @@ uint8_t epte_get_entry_emt(struct domain *d, unsigned long 
gfn, mfn_t mfn,
                   MTRR_TYPE_WRBACK;
 
     hmtrr_mtype = get_mtrr_type(&mtrr_state, (mfn_x(mfn) << PAGE_SHIFT));
-    return ((gmtrr_mtype <= hmtrr_mtype) ? gmtrr_mtype : hmtrr_mtype);
+
+    /* If both types match we're fine. */
+    if ( likely(gmtrr_mtype == hmtrr_mtype) )
+        return hmtrr_mtype;
+
+    /* If either type is UC, we have to go with that one. */
+    if ( gmtrr_mtype == MTRR_TYPE_UNCACHABLE ||
+         hmtrr_mtype == MTRR_TYPE_UNCACHABLE )
+        return MTRR_TYPE_UNCACHABLE;
+
+    /* If either type is WB, we have to go with the other one. */
+    if ( gmtrr_mtype == MTRR_TYPE_WRBACK )
+        return hmtrr_mtype;
+    if ( hmtrr_mtype == MTRR_TYPE_WRBACK )
+        return gmtrr_mtype;
+
+    /*
+     * At this point we have disagreeing WC, WT, or WP types. The only
+     * combination that can be cleanly resolved is WT:WP. The ones involving
+     * WC need to be converted to UC, both due to the memory ordering
+     * differences and because WC disallows reads to be cached (WT and WP
+     * permit this), while WT and WP require writes to go straight to memory
+     * (WC can buffer them).
+     */
+    if ( (gmtrr_mtype == MTRR_TYPE_WRTHROUGH &&
+          hmtrr_mtype == MTRR_TYPE_WRPROT) ||
+         (gmtrr_mtype == MTRR_TYPE_WRPROT &&
+          hmtrr_mtype == MTRR_TYPE_WRTHROUGH) )
+        return MTRR_TYPE_WRPROT;
+
+    return MTRR_TYPE_UNCACHABLE;
 }
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.4

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