[PATCH v3 5/5] x86/alternative: build time check feature is in range

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Ensure at build time the feature(s) used for the alternative blocks are in
range of the featureset.

No functional change intended, as all current usages are correct.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
Changes since v2:
 - s/__stringify/STR/.

Changes since v1:
 - New in this version.
---
 xen/arch/x86/include/asm/alternative-asm.h | 3 +++
 xen/arch/x86/include/asm/alternative.h     | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/xen/arch/x86/include/asm/alternative-asm.h 
b/xen/arch/x86/include/asm/alternative-asm.h
index 4092f5ba70a6..83e8594f0eaf 100644
--- a/xen/arch/x86/include/asm/alternative-asm.h
+++ b/xen/arch/x86/include/asm/alternative-asm.h
@@ -12,6 +12,9 @@
  * instruction. See apply_alternatives().
  */
 .macro altinstruction_entry orig, repl, feature, orig_len, repl_len, pad_len
+    .if \feature >= NCAPINTS * 32
+        .error "alternative feature outside of featureset range"
+    .endif
     .long \orig - .
     .long \repl - .
     .word \feature
diff --git a/xen/arch/x86/include/asm/alternative.h 
b/xen/arch/x86/include/asm/alternative.h
index 69555d781ef9..38472fb58e2d 100644
--- a/xen/arch/x86/include/asm/alternative.h
+++ b/xen/arch/x86/include/asm/alternative.h
@@ -7,6 +7,7 @@
 #include <xen/lib.h>
 #include <xen/stringify.h>
 #include <asm/asm-macros.h>
+#include <asm/cpufeatureset.h>
 
 struct __packed alt_instr {
     int32_t  orig_offset;   /* original instruction */
@@ -59,6 +60,9 @@ extern void alternative_branches(void);
                     alt_repl_len(n2)) "-" alt_orig_len)
 
 #define ALTINSTR_ENTRY(feature, num)                                    \
+        " .if " STR(feature) " >= " STR(NCAPINTS * 32) "\n"             \
+        " .error \"alternative feature outside of featureset range\"\n" \
+        " .endif\n"                                                     \
         " .long .LXEN%=_orig_s - .\n"             /* label           */ \
         " .long " alt_repl_s(num)" - .\n"         /* new instruction */ \
         " .word " __stringify(feature) "\n"       /* feature bit     */ \
-- 
2.46.0