[PATCH v1] Use CLOCK_MONOTONIC_RAW if available for get_clock().

[PATCH v1] Use CLOCK_MONOTONIC_RAW if available for get_clock().

[Joe Tanen]

CLOCK_MONOTONIC_RAW provides an unadjusted system clock on some platforms,
which is closer in spirit to providing a guest with a raw hardware clock than
CLOCK_MONOTONIC.

Using CLOCK_MONOTONIC_RAW also works around a current issue in OSX where
CLOCK_MONOTONIC has been observed to go backwards.

Since CLOCK_MONOTONIC_RAW might not be available on all platforms, revert to
using CLOCK_MONOTONIC if it is not present.

Signed-off-by: Joe Tanen <jtanen@fb.com>
---
 include/qemu/timer.h     |  3 ++-
 util/qemu-timer-common.c | 11 +++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/include/qemu/timer.h b/include/qemu/timer.h
index 88ef114689..fb8f5074df 100644
--- a/include/qemu/timer.h
+++ b/include/qemu/timer.h
@@ -828,12 +828,13 @@ static inline int64_t get_clock(void)
 #else
 
 extern int use_rt_clock;
+extern clockid_t rt_clock;
 
 static inline int64_t get_clock(void)
 {
     if (use_rt_clock) {
         struct timespec ts;
-        clock_gettime(CLOCK_MONOTONIC, &ts);
+        clock_gettime(rt_clock, &ts);
         return ts.tv_sec * 1000000000LL + ts.tv_nsec;
     } else {
         /* XXX: using gettimeofday leads to problems if the date
diff --git a/util/qemu-timer-common.c b/util/qemu-timer-common.c
index cc1326f726..5039c5406c 100644
--- a/util/qemu-timer-common.c
+++ b/util/qemu-timer-common.c
@@ -49,13 +49,24 @@ static void __attribute__((constructor)) init_get_clock(void)
 #else
 
 int use_rt_clock;
+clockid_t rt_clock;
 
 static void __attribute__((constructor)) init_get_clock(void)
 {
     struct timespec ts;
 
     use_rt_clock = 0;
+#if (defined(__APPLE__) || defined(__linux__)) && defined(CLOCK_MONOTONIC_RAW)
+    /* CLOCK_MONOTONIC_RAW is not available on all platforms or with all
+     * compiler flags.
+     */
+    if (clock_gettime(CLOCK_MONOTONIC_RAW, &ts) == 0) {
+        rt_clock = CLOCK_MONOTONIC_RAW;
+        use_rt_clock = 1;
+    } else
+#endif
     if (clock_gettime(CLOCK_MONOTONIC, &ts) == 0) {
+        rt_clock = CLOCK_MONOTONIC;
         use_rt_clock = 1;
     }
     clock_start = get_clock();
-- 
2.31.1

Re: [PATCH v1] Use CLOCK_MONOTONIC_RAW if available for get_clock().

[Peter Maydell]

On Thu, 30 Sept 2021 at 17:04, Joe Tanen <jtanen@fb.com> wrote:
>
> CLOCK_MONOTONIC_RAW provides an unadjusted system clock on some platforms,
> which is closer in spirit to providing a guest with a raw hardware clock than
> CLOCK_MONOTONIC.
>
> Using CLOCK_MONOTONIC_RAW also works around a current issue in OSX where
> CLOCK_MONOTONIC has been observed to go backwards.
>
> Since CLOCK_MONOTONIC_RAW might not be available on all platforms, revert to
> using CLOCK_MONOTONIC if it is not present.
>
> Signed-off-by: Joe Tanen <jtanen@fb.com>

I'm not sure we want to change behaviour everywhere to work
around an OSX bug, though...

Has this bug been reported to Apple ? Is there some kind of bug
report ID or URL we can quote in the commit message ?

-- PMM

Re: [PATCH v1] Use CLOCK_MONOTONIC_RAW if available for get_clock().

[Joe Tanen]

Peter,

Thanks for the quick response. I've informally socialized the issue and will update this thread when I get more information.

That aside, I'd find using CLOCK_MONOTONIC_RAW valuable if, e.g., I wanted test an NTP daemon inside of a guest and didn't want the host providing an already-adjusted timebase. Would the behavior from my patch be more appropriate as a command-line option?

v/r
Joe

On 9/30/21, 12:11 PM, "Peter Maydell" <peter.maydell@linaro.org> wrote:

    On Thu, 30 Sept 2021 at 17:04, Joe Tanen <jtanen@fb.com> wrote:
    >
    > CLOCK_MONOTONIC_RAW provides an unadjusted system clock on some platforms,
    > which is closer in spirit to providing a guest with a raw hardware clock than
    > CLOCK_MONOTONIC.
    >
    > Using CLOCK_MONOTONIC_RAW also works around a current issue in OSX where
    > CLOCK_MONOTONIC has been observed to go backwards.
    >
    > Since CLOCK_MONOTONIC_RAW might not be available on all platforms, revert to
    > using CLOCK_MONOTONIC if it is not present.
    >
    > Signed-off-by: Joe Tanen <jtanen@fb.com>

    I'm not sure we want to change behaviour everywhere to work
    around an OSX bug, though...

    Has this bug been reported to Apple ? Is there some kind of bug
    report ID or URL we can quote in the commit message ?

    -- PMM