[Xen-devel] [PATCH] public/xen.h: update the comment explaining 'Wallclock time'

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

Since commit 0629adfd80e "Actually set a HVM domain's time offset when it
sets the RTC", the comment in the public header has been misleading, since
it claims that wallclock time is only updated by control software.
Moreover, the comments stating that wc_sec and wc_nsec are seconds and
nanoseconds (respectively) in UTC since the Unix epoch are bogus. Their
values are adjusted by the domain's time_offset_seconds value, which is
updated by a guest write to the emulated RTC and hence the wallclock
timezone is under guest control.

This patch attempts to bring the comment in line with reality whilst
keeping it reasonably short.

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
---
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
Cc: Wei Liu <wl@xxxxxxx>
---
 xen/include/public/xen.h | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/xen/include/public/xen.h b/xen/include/public/xen.h
index cef65c38e7..2755c5ad54 100644
--- a/xen/include/public/xen.h
+++ b/xen/include/public/xen.h
@@ -775,12 +775,17 @@ struct shared_info {
     xen_ulong_t evtchn_mask[sizeof(xen_ulong_t) * 8];
 
     /*
-     * Wallclock time: updated only by control software. Guests should base
-     * their gettimeofday() syscall on this wallclock-base value.
+     * Wallclock time: updated by control software or RTC emulation.
+     * Guests should base their gettimeofday() syscall on this
+     * wallclock-base value.
+     * The valus of wc_sec and wc_nsec are offsets from the Unix epoch
+     * adjusted by the domain's 'time offset' (in seconds) as set either
+     * by XEN_DOMCTL_settimeoffset, or adjusted via a guest write to the
+     * emulated RTC.
      */
     uint32_t wc_version;      /* Version counter: see vcpu_time_info_t. */
-    uint32_t wc_sec;          /* Secs  00:00:00 UTC, Jan 1, 1970.  */
-    uint32_t wc_nsec;         /* Nsecs 00:00:00 UTC, Jan 1, 1970.  */
+    uint32_t wc_sec;
+    uint32_t wc_nsec;
 #if !defined(__i386__)
     uint32_t wc_sec_hi;
 # define xen_wc_sec_hi wc_sec_hi
-- 
2.20.1.2.gb21ebb671


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