x86/thunk: (Mis)align __x86_indirect_thunk_* to mitigate ITS

The Indirect Target Selection speculative vulnerability means that indirect
branches (including RETs) are unsafe when in the first half of a cacheline.

Arrange for __x86_indirect_thunk_* to always be in the second half.

This is part of XSA-469 / CVE-2024-28956

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit d293cc9da9)

xen/arch/x86/indirect-thunk.S

@@ -11,6 +11,10 @@
 
 #include <asm/asm_defns.h>
 
+/* Alignment is dealt with explicitly here; override the respective macro. */
+#undef SYM_ALIGN
+#define SYM_ALIGN(align...)
+
 .macro IND_THUNK_RETPOLINE reg:req
         call 1f
         int3
@@ -35,6 +39,16 @@
 .macro GEN_INDIRECT_THUNK reg:req
         .section .text.__x86_indirect_thunk_\reg, "ax", @progbits
 
+        /*
+         * The Indirect Target Selection speculative vulnerability means that
+         * indirect branches (including RETs) are unsafe when in the first
+         * half of a cacheline.  Arrange for them to be in the second half.
+         *
+         * Align to 64, then skip 32.
+         */
+        .balign 64
+        .fill 32, 1, 0xcc
+
 FUNC(__x86_indirect_thunk_\reg)
         ALTERNATIVE_2 __stringify(IND_THUNK_RETPOLINE \reg),              \
         __stringify(IND_THUNK_LFENCE \reg), X86_FEATURE_IND_THUNK_LFENCE, \