[Qemu-devel] [PATCH 1/2] Correct alarm deadline computation

[Qemu-devel] [PATCH 1/2] Correct alarm deadline computation

[Paolo Bonzini]

When the QEMU_CLOCK_HOST clock was added, computation of its
deadline was added to qemu_next_deadline, which is correct but
incomplete.

I noticed this while trying to make sense of the rules whereby
qemu_next_deadline_dyntick is computed, which miss QEMU_CLOCK_HOST
when use_icount is true.  Looking at the history showed this to
be just an oversight, as the next patch shows clearly.

This patch inlines qemu_next_deadline into qemu_next_deadline_dyntick,
and corrects the logic so that only QEMU_CLOCK_VIRTUAL is skipped for
use_icount == true.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
---
 qemu-timer.c |   15 +++++++++++----
 1 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/qemu-timer.c b/qemu-timer.c
index db1ec49..174fd0c 100644
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -724,11 +724,18 @@ static uint64_t qemu_next_deadline_dyntick(void)
     int64_t delta;
     int64_t rtdelta;
 
-    if (use_icount)
+    if (!use_icount && active_timers[QEMU_CLOCK_VIRTUAL]) {
+        delta = active_timers[QEMU_CLOCK_VIRTUAL]->expire_time -
+                     qemu_get_clock(vm_clock);
+    } else {
         delta = INT32_MAX;
-    else
-        delta = (qemu_next_deadline() + 999) / 1000;
-
+    }
+    if (active_timers[QEMU_CLOCK_HOST]) {
+        int64_t hdelta = active_timers[QEMU_CLOCK_HOST]->expire_time -
+                 qemu_get_clock(host_clock);
+        if (hdelta < delta)
+            delta = hdelta;
+    }
     if (active_timers[QEMU_CLOCK_REALTIME]) {
         rtdelta = (active_timers[QEMU_CLOCK_REALTIME]->expire_time -
                  qemu_get_clock(rt_clock))*1000;
-- 
1.7.3.4

[Qemu-devel] [PATCH 2/2] Unify alarm deadline computation

[Paolo Bonzini]

Now that qemu_next_deadline_dyntick uses the correct formula, we can
simplify the code in host_alarm_handler and eliminate useless duplication.

Also, qemu_next_deadline_dyntick is always used in signed arithmetic,
so change its type.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 qemu-timer.c |   28 +++++++++++-----------------
 roms/vgabios |    2 +-
 2 files changed, 12 insertions(+), 18 deletions(-)

diff --git a/qemu-timer.c b/qemu-timer.c
index 174fd0c..1842d7a 100644
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -635,6 +635,8 @@ void qemu_run_all_timers(void)
     qemu_run_timers(host_clock);
 }
 
+static int64_t qemu_next_alarm_deadline(void);
+
 #ifdef _WIN32
 static void CALLBACK host_alarm_handler(UINT uTimerID, UINT uMsg,
                                         DWORD_PTR dwUser, DWORD_PTR dw1,
@@ -677,14 +679,7 @@ static void host_alarm_handler(int host_signum)
     }
 #endif
     if (alarm_has_dynticks(t) ||
-        (!use_icount &&
-            qemu_timer_expired(active_timers[QEMU_CLOCK_VIRTUAL],
-                               qemu_get_clock(vm_clock))) ||
-        qemu_timer_expired(active_timers[QEMU_CLOCK_REALTIME],
-                           qemu_get_clock(rt_clock)) ||
-        qemu_timer_expired(active_timers[QEMU_CLOCK_HOST],
-                           qemu_get_clock(host_clock))) {
-
+        qemu_next_alarm_deadline () <= 0) {
         t->expired = alarm_has_dynticks(t);
         t->pending = 1;
         qemu_notify_event();
@@ -715,11 +710,7 @@ int64_t qemu_next_deadline(void)
 
 #ifndef _WIN32
 
-#if defined(__linux__)
-
-#define RTC_FREQ 1024
-
-static uint64_t qemu_next_deadline_dyntick(void)
+static int64_t qemu_next_alarm_deadline(void)
 {
     int64_t delta;
     int64_t rtdelta;
@@ -743,12 +734,13 @@ static uint64_t qemu_next_deadline_dyntick(void)
             delta = rtdelta;
     }
 
-    if (delta < MIN_TIMER_REARM_US)
-        delta = MIN_TIMER_REARM_US;
-
     return delta;
 }
 
+#if defined(__linux__)
+
+#define RTC_FREQ 1024
+
 static void enable_sigio_timer(int fd)
 {
     struct sigaction act;
@@ -903,7 +895,9 @@ static void dynticks_rearm_timer(struct qemu_alarm_timer *t)
         !active_timers[QEMU_CLOCK_HOST])
         return;
 
-    nearest_delta_us = qemu_next_deadline_dyntick();
+    nearest_delta_us = qemu_next_alarm_deadline();
+    if (nearest_delta_us < MIN_TIMER_REARM_US)
+        nearest_delta_us = MIN_TIMER_REARM_US;
 
     /* check whether a timer is already running */
     if (timer_gettime(host_timer, &timeout)) {
-- 
1.7.3.4

Re: [Qemu-devel] [PATCH 1/2] Correct alarm deadline computation

[Aurelien Jarno]

On Sat, Jan 29, 2011 at 12:00:47PM +0100, Paolo Bonzini wrote:
> When the QEMU_CLOCK_HOST clock was added, computation of its
> deadline was added to qemu_next_deadline, which is correct but
> incomplete.
> 
> I noticed this while trying to make sense of the rules whereby
> qemu_next_deadline_dyntick is computed, which miss QEMU_CLOCK_HOST
> when use_icount is true.  Looking at the history showed this to
> be just an oversight, as the next patch shows clearly.
> 
> This patch inlines qemu_next_deadline into qemu_next_deadline_dyntick,
> and corrects the logic so that only QEMU_CLOCK_VIRTUAL is skipped for
> use_icount == true.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Jan Kiszka <jan.kiszka@siemens.com>
> ---
>  qemu-timer.c |   15 +++++++++++----
>  1 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/qemu-timer.c b/qemu-timer.c
> index db1ec49..174fd0c 100644
> --- a/qemu-timer.c
> +++ b/qemu-timer.c
> @@ -724,11 +724,18 @@ static uint64_t qemu_next_deadline_dyntick(void)
>      int64_t delta;
>      int64_t rtdelta;
>  
> -    if (use_icount)
> +    if (!use_icount && active_timers[QEMU_CLOCK_VIRTUAL]) {
> +        delta = active_timers[QEMU_CLOCK_VIRTUAL]->expire_time -
> +                     qemu_get_clock(vm_clock);
> +    } else {
>          delta = INT32_MAX;
> -    else
> -        delta = (qemu_next_deadline() + 999) / 1000;
> -

This computation handled the fact that the value is returned in ns, and
we want us.

> +    }
> +    if (active_timers[QEMU_CLOCK_HOST]) {
> +        int64_t hdelta = active_timers[QEMU_CLOCK_HOST]->expire_time -
> +                 qemu_get_clock(host_clock);
> +        if (hdelta < delta)
> +            delta = hdelta;
> +    }

And this doesn't appear anymore after your changes.

>      if (active_timers[QEMU_CLOCK_REALTIME]) {
>          rtdelta = (active_timers[QEMU_CLOCK_REALTIME]->expire_time -
>                   qemu_get_clock(rt_clock))*1000;

Here we multiply because the value is returned in ms and we want us.

IMHO we should just use qemu_get_clock_ns() instead, replace
MIN_TIMER_REARM_US by MIN_TIMER_REARM_NS, and return a ns value
instead. Of course dynticks_rearm_timer() has to be adjusted, but it
should not be problematic given it uses ns internally.

Otherwise good catch, the issue is real, and the way to fix it looks
correct.

-- 
Aurelien Jarno	                        GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net