Allow setting the used wallclock from the command line.  When the option is set
to a value different than `auto` the probing is bypassed and the selected
implementation is used (as long as it's available).

The `xen` and `efi` options require being booted as a Xen guest (with Xen guest
supported built-in) or from UEFI firmware.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Changes since v3:
 - Note xen option is only available if Xen guest support it built.
 - Fix typo.
---
 docs/misc/xen-command-line.pandoc | 19 ++++++++++++++++++
 xen/arch/x86/time.c               | 33 ++++++++++++++++++++++++++++++-
 2 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index 0a66e1ee2d7e..1944b9d8eb9d 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2821,6 +2821,25 @@ vwfi to `native` reduces irq latency significantly. It can also lead to
 suboptimal scheduling decisions, but only when the system is
 oversubscribed (i.e., in total there are more vCPUs than pCPUs).
 
+### wallclock (x86)
+> `= auto | xen | cmos | efi`
+
+> Default: `auto`
+
+Allow forcing the usage of a specific wallclock source.
+
+ * `auto` let the hypervisor select the clocksource based on internal
+   heuristics.
+
+ * `xen` force usage of the Xen shared_info wallclock when booted as a Xen
+   guest.  This option is only available if the hypervisor was compiled with
+   `CONFIG_XEN_GUEST` enabled.
+
+ * `cmos` force usage of the CMOS RTC wallclock.
+
+ * `efi` force usage of the EFI_GET_TIME run-time method when booted from EFI
+   firmware.
+
 ### watchdog (x86)
 > `= force | <boolean>`
 
diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 9ebeee61b987..52944b8fbfde 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1553,6 +1553,36 @@ static const char *__init wallclock_type_to_string(void)
     return "";
 }
 
+static int __init cf_check parse_wallclock(const char *arg)
+{
+    if ( !arg )
+        return -EINVAL;
+
+    if ( !strcmp("auto", arg) )
+        wallclock_source = WALLCLOCK_UNSET;
+    else if ( !strcmp("xen", arg) )
+    {
+        if ( !xen_guest )
+            return -EINVAL;
+
+        wallclock_source = WALLCLOCK_XEN;
+    }
+    else if ( !strcmp("cmos", arg) )
+        wallclock_source = WALLCLOCK_CMOS;
+    else if ( !strcmp("efi", arg) )
+    {
+        if ( !efi_enabled(EFI_RS) )
+            return -EINVAL;
+
+        wallclock_source = WALLCLOCK_EFI;
+    }
+    else
+        return -EINVAL;
+
+    return 0;
+}
+custom_param("wallclock", parse_wallclock);
+
 static void __init probe_wallclock(void)
 {
     ASSERT(wallclock_source == WALLCLOCK_UNSET);
@@ -2528,7 +2558,8 @@ int __init init_xen_time(void)
 
     open_softirq(TIME_CALIBRATE_SOFTIRQ, local_time_calibration);
 
-    probe_wallclock();
+    if ( wallclock_source == WALLCLOCK_UNSET )
+        probe_wallclock();
 
     printk(XENLOG_INFO "Wallclock source: %s\n", wallclock_type_to_string());
 
-- 
2.46.0