x86/time: prefer CMOS over EFI_GET_TIME

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>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-By: Oleksii Kurochko<oleksii.kurochko@gmail.com>
Release-Acked-by: Oleksii Kurochko<oleksii.kurochko@gmail.com>

CHANGELOG.md

@@ -12,6 +12,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    leaving this to the guest kernel to do in guest context.
  - On x86:
    - Prefer ACPI reboot over UEFI ResetSystem() run time service call.
+   - Prefer CMOS over EFI_GET_TIME as time source.
    - Switched the xAPIC flat driver to use physical destination mode for external
      interrupts instead of logical destination mode.
 
@@ -24,6 +25,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    - Support for LLC (Last Level Cache) coloring.
  - On x86:
    - xl suspend/resume subcommands.
+   - `wallclock` command line option to select time source.
 
 ### Removed
  - On x86:

xen/arch/x86/time.c

@@ -1592,16 +1592,16 @@ static void __init probe_wallclock(void)
         wallclock_source = WALLCLOCK_XEN;
         return;
     }
-    if ( efi_enabled(EFI_RS) && efi_get_time() )
-    {
-        wallclock_source = WALLCLOCK_EFI;
-        return;
-    }
     if ( cmos_rtc_probe() )
     {
         wallclock_source = WALLCLOCK_CMOS;
         return;
     }
+    if ( efi_enabled(EFI_RS) && efi_get_time() )
+    {
+        wallclock_source = WALLCLOCK_EFI;
+        return;
+    }
 
     panic("No usable wallclock found, probed:%s%s%s\n%s",
           !opt_cmos_rtc_probe && !efi_enabled(EFI_RS) ? " None" : "",