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>

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8507e6556a56ecbf58a6e4fee8a99354719eef7b..1de1d1eca17fc9021cbe04a843d4d0b392d59bb2 100644 (file)
--- a/CHANGELOG.md
+++ b/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:

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 00545c45fde47fee7c911cf7b70e4ba5c973cbd8..59129f419d3cb45a53eb1ef8330688b0c7a7159f 100644 (file)
--- 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_rtc_probe() )
     {
-        wallclock_source = WALLCLOCK_EFI;
+        wallclock_source = WALLCLOCK_CMOS;
         return;
     }
-    if ( cmos_rtc_probe() )
+    if ( efi_enabled(EFI_RS) && efi_get_time() )
     {
-        wallclock_source = WALLCLOCK_CMOS;
+        wallclock_source = WALLCLOCK_EFI;
         return;
     }