xen/riscv: introduce ANDN_INSN

RISC-V does a conditional toolchain for the Zbb extension
(xen/arch/riscv/rules.mk), but unconditionally uses the
ANDN instruction in emulate_xchg_1_2().

Fixes: 51dabd6312 ("xen/riscv: introduce cmpxchg.h")
Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Suggested-By: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/riscv/include/asm/cmpxchg.h

@@ -18,6 +18,19 @@
         : "r" (new) \
         : "memory" );
 
+/*
+ * To not face an issue that gas doesn't understand ANDN instruction
+ * it is encoded using .insn directive.
+ */
+#ifdef __riscv_zbb
+#define ANDN_INSN(rd, rs1, rs2)                 \
+    ".insn r OP, 0x7, 0x20, " rd ", " rs1 ", " rs2 "\n"
+#else
+#define ANDN_INSN(rd, rs1, rs2)                 \
+    "not " rd ", " rs2 "\n"                     \
+    "and " rd ", " rs1 ", " rd "\n"
+#endif
+
 /*
  * For LR and SC, the A extension requires that the address held in rs1 be
  * naturally aligned to the size of the operand (i.e., eight-byte aligned
@@ -48,7 +61,7 @@
     \
     asm volatile ( \
         "0: lr.w" lr_sfx " %[old], %[ptr_]\n" \
-        "   andn  %[scratch], %[old], %[mask]\n" \
+        ANDN_INSN("%[scratch]", "%[old]", "%[mask]") \
         "   or   %[scratch], %[scratch], %z[new_]\n" \
         "   sc.w" sc_sfx " %[scratch], %[scratch], %[ptr_]\n" \
         "   bnez %[scratch], 0b\n" \