The EFI_GET_TIME implementation is well known to be broken for many firmware
implementations, for Xen the result on such implementations are:

----[ Xen-4.19-unstable  x86_64  debug=y  Tainted:   C    ]----
CPU:    0
RIP:    e008:[<0000000062ccfa70>] 0000000062ccfa70
[...]
Xen call trace:
   [<0000000062ccfa70>] R 0000000062ccfa70
   [<00000000732e9a3f>] S 00000000732e9a3f
   [<ffff82d04034f34f>] F arch/x86/time.c#get_cmos_time+0x1b3/0x26e
   [<ffff82d04045926f>] F init_xen_time+0x28/0xa4
   [<ffff82d040454bc4>] F __start_xen+0x1ee7/0x2578
   [<ffff82d040203334>] F __high_start+0x94/0xa0

Pagetable walk from 0000000062ccfa70:
 L4[0x000] = 000000207ef1c063 ffffffffffffffff
 L3[0x001] = 000000005d6c0063 ffffffffffffffff
 L2[0x116] = 8000000062c001e3 ffffffffffffffff (PSE)

****************************************
Panic on CPU 0:
FATAL PAGE FAULT
[error_code=0011]
Faulting linear address: 0000000062ccfa70
****************************************

Swap the preference to default to CMOS first, and EFI later, in an attempt to
use EFI_GET_TIME as a last resort option only.  Note that Linux for example
doesn't allow calling the get_time method, and instead provides a dummy handler
that unconditionally returns EFI_UNSUPPORTED on x86-64.

Such change in the preferences requires some re-arranging of the function
logic, so that panic messages with workaround suggestions are suitably printed.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Changes since v2:
 - Updated to match previous changes.
---
 xen/arch/x86/time.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 52944b8fbfde..788a12d52fc3 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1592,14 +1592,14 @@ static void __init probe_wallclock(void)
         wallclock_source = WALLCLOCK_XEN;
         return;
     }
-    if ( efi_enabled(EFI_RS) && efi_get_time() )
+    if ( cmos_probe() )
     {
-        wallclock_source = WALLCLOCK_EFI;
+        wallclock_source = WALLCLOCK_CMOS;
         return;
     }
-    if ( cmos_probe() )
+    if ( efi_enabled(EFI_RS) && efi_get_time() )
     {
-        wallclock_source = WALLCLOCK_CMOS;
+        wallclock_source = WALLCLOCK_EFI;
         return;
     }
 
-- 
2.46.0