[PATCH 6/9] xen/arm64: entry: Don't jump outside of an alternative

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

The instruction CBNZ can only jump to a pc-relative that is in the
range +/- 1MB.

Alternative instructions replacement are living in a separate
subsection of the init section. This is usually placed towards
the end of the linker. Whereas text is towards the beginning.

While today Xen is quite small (~1MB), it could grow up to
2MB in the current setup. So there is no guarantee that the
target address in the text section will be within the range +/-
1MB of the CBNZ in alternative section.

The easiest solution is to have the target address within the
same section of the alternative. This means that we need to
duplicate a couple of instructions.

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

----

I couldn't come up with a solution that would not change the number
of instructions executed in the entry path.
---
 xen/arch/arm/arm64/entry.S | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
index 95f1a9268419..492591fdef54 100644
--- a/xen/arch/arm/arm64/entry.S
+++ b/xen/arch/arm/arm64/entry.S
@@ -242,13 +242,24 @@
         msr     daifclr, \iflags
         bl      enter_hypervisor_from_guest
 
+        /*
+         * CBNZ can only address an offset of +/- 1MB. This means, it is
+         * not possible to jump outside of an alternative because
+         * the .text section and .altinstr_replacement may be further
+         * appart. The easiest way is to duplicate the few instructions
+         * that need to be skipped.
+         */
         alternative_if SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT
-        cbnz    x19, 1f
-        alternative_else_nop_endif
-
-        mov     x0, sp
-        bl      do_trap_\trap
+        cbnz      x19, 1f
+        mov       x0, sp
+        bl        do_trap_\trap
 1:
+        alternative_else
+        nop
+        mov       x0, sp
+        bl        do_trap_\trap
+        alternative_endif
+
         exit    hyp=0, compat=\compat
         .endm
 
-- 
2.40.1