[PATCH v5 07/10] x86: Derive XEN_MSR_PAT from its individual entries

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

This avoids it being a magic constant that is difficult for humans to
decode.  Use BUILD_BUG_ON to check that the old and new values are
identical.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Changes since v4:
- Explain that changing XEN_MSR_PAT breaks guests that rely on the API
  in xen.h instead of reading the PAT from Xen.
---
 xen/arch/x86/include/asm/processor.h |  9 ++++++++-
 xen/arch/x86/mm.c                    | 11 +++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/include/asm/processor.h 
b/xen/arch/x86/include/asm/processor.h
index 
8e2816fae9b97bd4e153a30cc3802971fe0355af..60b902060914584957db8afa5c7c1e6abdad4d13
 100644
--- a/xen/arch/x86/include/asm/processor.h
+++ b/xen/arch/x86/include/asm/processor.h
@@ -96,7 +96,14 @@
  * Host IA32_CR_PAT value to cover all memory types.  This is not the default
  * MSR_PAT value, and is an ABI with PV guests.
  */
-#define XEN_MSR_PAT _AC(0x050100070406, ULL)
+#define XEN_MSR_PAT ((_AC(X86_MT_WB,  ULL) << 0x00) | \
+                     (_AC(X86_MT_WT,  ULL) << 0x08) | \
+                     (_AC(X86_MT_UCM, ULL) << 0x10) | \
+                     (_AC(X86_MT_UC,  ULL) << 0x18) | \
+                     (_AC(X86_MT_WC,  ULL) << 0x20) | \
+                     (_AC(X86_MT_WP,  ULL) << 0x28) | \
+                     (_AC(X86_MT_UC,  ULL) << 0x30) | \
+                     (_AC(X86_MT_UC,  ULL) << 0x38))
 
 #ifndef __ASSEMBLY__
 
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 
dba6c77ef2f7ed7fcb7f7e526583ccadd35e62cc..b40a575b61418ea1137299e68b64f7efd9efeced
 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -6352,6 +6352,17 @@ unsigned long get_upper_mfn_bound(void)
     return min(max_mfn, 1UL << (paddr_bits - PAGE_SHIFT)) - 1;
 }
 
+static void __init __maybe_unused build_assertions(void)
+{
+    /*
+     * If this trips, any guest that blindly rely on the public API in xen.h
+     * (instead of reading the PAT from Xen, as Linux 3.19+ does) will be
+     * broken.  Furthermore, live migration of PV guests between Xen versions
+     * using different PATs will not work.
+     */
+    BUILD_BUG_ON(XEN_MSR_PAT != 0x050100070406ULL);
+}
+
 /*
  * Local variables:
  * mode: C
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab