[PATCH v2 12/14] x86/mm: make code robust to future PAT changes

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

It may be desirable to change Xen's PAT for various reasons.  This
requires changes to several _PAGE_* macros as well.  Add static
assertions to check that XEN_MSR_PAT is consistent with the _PAGE_*
macros, and that _PAGE_WB is 0 as required by Linux.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
---
 xen/arch/x86/mm.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 
a8def47aa3bf9770576c62a190032d45d63dd86e..7fb1a0f91910952640378f316a68096a08895b37
 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -6357,8 +6357,44 @@ unsigned long get_upper_mfn_bound(void)
 
 static void __init __maybe_unused build_assertions(void)
 {
+    /* A bunch of static assertions to check that the XEN_MSR_PAT is valid
+     * and consistent with the _PAGE_* macros */
     BUILD_BUG_ON(XEN_MSR_PAT != 0x050100070406ULL &&
                  "wrong XEN_MSR_PAT breaks PV guests");
+    BUILD_BUG_ON(_PAGE_WB && "Linux requires _PAGE_WB to be 0");
+#define PAT_VALUE(v) (0xFF & (XEN_MSR_PAT >> (8 * (v))))
+#define BAD_VALUE(v) ((v) < 0 || (v) > 7 ||                                    
\
+                      (v) == X86_MT_RESERVED_1 || (v) == X86_MT_RESERVED_2)
+#define BAD_PAT_VALUE(v) BUILD_BUG_ON(BAD_VALUE(PAT_VALUE(v)))
+    BAD_PAT_VALUE(0);
+    BAD_PAT_VALUE(1);
+    BAD_PAT_VALUE(2);
+    BAD_PAT_VALUE(3);
+    BAD_PAT_VALUE(4);
+    BAD_PAT_VALUE(5);
+    BAD_PAT_VALUE(6);
+    BAD_PAT_VALUE(7);
+#undef BAD_PAT_VALUE
+#undef BAD_VALUE
+#define PAT_SHIFT(page_value) (((page_value) & _PAGE_PAT) >> 5 |               
\
+                               ((page_value) & (_PAGE_PCD | _PAGE_PWT)) >> 3)
+#define CHECK_PAGE_VALUE(page_value) do {                                      
\
+    /* Check that the _PAGE_* macros only use bits from PAGE_CACHE_ATTRS */    
\
+    BUILD_BUG_ON(((_PAGE_##page_value) & PAGE_CACHE_ATTRS) !=                  
\
+                  (_PAGE_##page_value));                                       
\
+    /* Check that the _PAGE_* are consistent with XEN_MSR_PAT */               
\
+    BUILD_BUG_ON(PAT_VALUE(PAT_SHIFT(_PAGE_##page_value)) !=                   
\
+                 (X86_MT_##page_value));                                       
\
+} while (0)
+    CHECK_PAGE_VALUE(WT);
+    CHECK_PAGE_VALUE(WB);
+    CHECK_PAGE_VALUE(WC);
+    CHECK_PAGE_VALUE(UC);
+    CHECK_PAGE_VALUE(UCM);
+    CHECK_PAGE_VALUE(WP);
+#undef CHECK_PAGE_VALUE
+#undef PAT_SHIFT
+#undef PAT_VALUE
 }
 
 /*
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab