[PATCH v4 08/10] 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>
---
Changes since v3:
- Refactor some macros
- Avoid including a string literal in BUILD_BUG_ON

 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 
eeaa9760d2aa38b5efa2cbf9d99290e7c432ddfe..1bba833aecf636079bb60c45e34cdce3d4ddbba5
 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -6364,6 +6364,42 @@ static void __init __maybe_unused build_assertions(void)
      * broken.
      */
     BUILD_BUG_ON(XEN_MSR_PAT != 0x050100070406ULL);
+    /* A bunch of static assertions to check that the XEN_MSR_PAT is valid
+     * and consistent with the _PAGE_* macros */
+    BUILD_BUG_ON(_PAGE_WB);
+#define PAT_VALUE(v) (0xFF & (XEN_MSR_PAT >> (v)))
+#define BAD_VALUE(v) ((v) < 0 || (v) > 7 ||                                    
\
+                      (v) == X86_MT_RSVD_2 || (v) == X86_MT_RSVD_3)
+#define BAD_PAT_VALUE(v) BUILD_BUG_ON(BAD_VALUE(PAT_VALUE(8 * (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) >> 2) |             
\
+                               ((page_value) & (_PAGE_PCD | _PAGE_PWT)))
+#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