x86/alt: Introduce ALTERNATIVE{,_2} macros

To help creating alternative frames in assembly.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 4711428f5e
master date: 2018-01-05 19:57:07 +0000

xen/include/asm-x86/alternative-asm.h

@@ -17,6 +17,52 @@
     .byte \alt_len
 .endm
 
+.macro ALTERNATIVE oldinstr, newinstr, feature
+.Lold_start_\@:
+    \oldinstr
+.Lold_end_\@:
+
+    .pushsection .altinstructions, "a", @progbits
+    altinstruction_entry .Lold_start_\@, .Lnew_start_\@, \feature, \
+        (.Lold_end_\@ - .Lold_start_\@), (.Lnew_end_\@ - .Lnew_start_\@)
+
+    .section .discard, "a", @progbits
+    /* Assembler-time check that \newinstr isn't longer than \oldinstr. */
+    .byte 0xff + (.Lnew_end_\@ - .Lnew_start_\@) - (.Lold_end_\@ - .Lold_start_\@)
+
+    .section .altinstr_replacement, "ax", @progbits
+.Lnew_start_\@:
+    \newinstr
+.Lnew_end_\@:
+    .popsection
+.endm
+
+.macro ALTERNATIVE_2 oldinstr, newinstr1, feature1, newinstr2, feature2
+.Lold_start_\@:
+    \oldinstr
+.Lold_end_\@:
+
+    .pushsection .altinstructions, "a", @progbits
+    altinstruction_entry .Lold_start_\@, .Lnew1_start_\@, \feature1, \
+        (.Lold_end_\@ - .Lold_start_\@), (.Lnew1_end_\@ - .Lnew1_start_\@)
+    altinstruction_entry .Lold_start_\@, .Lnew2_start_\@, \feature2, \
+        (.Lold_end_\@ - .Lold_start_\@), (.Lnew2_end_\@ - .Lnew2_start_\@)
+
+    .section .discard, "a", @progbits
+    /* Assembler-time check that \newinstr{1,2} aren't longer than \oldinstr. */
+    .byte 0xff + (.Lnew1_end_\@ - .Lnew1_start_\@) - (.Lold_end_\@ - .Lold_start_\@)
+    .byte 0xff + (.Lnew2_end_\@ - .Lnew2_start_\@) - (.Lold_end_\@ - .Lold_start_\@)
+
+    .section .altinstr_replacement, "ax", @progbits
+.Lnew1_start_\@:
+    \newinstr1
+.Lnew1_end_\@:
+.Lnew2_start_\@:
+    \newinstr2
+.Lnew2_end_\@:
+    .popsection
+.endm
+
 #endif /* __ASSEMBLY__ */
 #endif /* _ASM_X86_ALTERNATIVE_ASM_H_ */