[PATCH] xen/arm64: entry: Actually skip do_trap_*() when an SError is triggered

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

For SErrors, we support two configurations:
  * Every SErrors will result to a panic in Xen
  * We will forward SErrors triggered by a VM back to itself

For the latter case, we want to skip the call to do_trap_*() because the PC
was already adjusted.

However, the alternative used to decide between the two configurations
is inverted. This would result to the VM corrupting itself if:
  * x19 is non-zero in the panic case
  * advance PC too much in the second case

Solve the issue by switch from alternative_if to alternative_if_not.

Fixes: a458d3bd0d25 ("xen/arm: entry: Ensure the guest state is synced when 
receiving a vSError")
Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

----

This is a candidate to be backported to all supported tree.

I don't have a setup where I can easily inject SError. But this was tested
by setting x19 to 1 just before the first alternative and use "serrors=panic".

Before this patch, Linux would get stuck.
---
 xen/arch/arm/arm64/entry.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
index 6251135ebdd2..fab10f8a0d26 100644
--- a/xen/arch/arm/arm64/entry.S
+++ b/xen/arch/arm/arm64/entry.S
@@ -259,7 +259,7 @@
          * apart. The easiest way is to duplicate the few instructions
          * that need to be skipped.
          */
-        alternative_if SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT
+        alternative_if_not SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT
         cbnz      x19, 1f
         mov       x0, sp
         bl        do_trap_\trap
-- 
2.40.1