[Qemu-devel] [PATCH v2 1/3] icount: implement a new icount_sleep mode toggleing real-time cpu sleep

[Victor CLEMENT <victor.clement@openwide.fr>]

When the icount_sleep mode is disabled, the QEMU_VIRTUAL_CLOCK runs at the
maximum possible speed by warping the sleep times of the virtual cpu to the
soonest clock deadline. The virtual clock will be updated only according
the instruction counter.

Signed-off-by: Victor CLEMENT <victor.clement@openwide.fr>
---
 cpus.c | 70 +++++++++++++++++++++++++++++++++++++++++-------------------------
 1 file changed, 44 insertions(+), 26 deletions(-)

diff --git a/cpus.c b/cpus.c
index de6469f..688eb73 100644
--- a/cpus.c
+++ b/cpus.c
@@ -105,6 +105,7 @@ static bool all_cpu_threads_idle(void)
 
 /* Protected by TimersState seqlock */
 
+static bool icount_sleep = true;
 static int64_t vm_clock_warp_start = -1;
 /* Conversion factor from emulated instructions to virtual clock ticks.  */
 static int icount_time_shift;
@@ -393,15 +394,18 @@ void qemu_clock_warp(QEMUClockType type)
         return;
     }
 
-    /*
-     * If the CPUs have been sleeping, advance QEMU_CLOCK_VIRTUAL timer now.
-     * This ensures that the deadline for the timer is computed correctly below.
-     * This also makes sure that the insn counter is synchronized before the
-     * CPU starts running, in case the CPU is woken by an event other than
-     * the earliest QEMU_CLOCK_VIRTUAL timer.
-     */
-    icount_warp_rt(NULL);
-    timer_del(icount_warp_timer);
+    if (icount_sleep) {
+        /*
+         * If the CPUs have been sleeping, advance QEMU_CLOCK_VIRTUAL timer now.
+         * This ensures that the deadline for the timer is computed correctly
+         * below.
+         * This also makes sure that the insn counter is synchronized before
+         * the CPU starts running, in case the CPU is woken by an event other
+         * than the earliest QEMU_CLOCK_VIRTUAL timer.
+         */
+        icount_warp_rt(NULL);
+        timer_del(icount_warp_timer);
+    }
     if (!all_cpu_threads_idle()) {
         return;
     }
@@ -425,23 +429,35 @@ void qemu_clock_warp(QEMUClockType type)
          * interrupt to wake it up, but the interrupt never comes because
          * the vCPU isn't running any insns and thus doesn't advance the
          * QEMU_CLOCK_VIRTUAL.
-         *
-         * An extreme solution for this problem would be to never let VCPUs
-         * sleep in icount mode if there is a pending QEMU_CLOCK_VIRTUAL
-         * timer; rather time could just advance to the next QEMU_CLOCK_VIRTUAL
-         * event.  Instead, we do stop VCPUs and only advance QEMU_CLOCK_VIRTUAL
-         * after some "real" time, (related to the time left until the next
-         * event) has passed. The QEMU_CLOCK_VIRTUAL_RT clock will do this.
-         * This avoids that the warps are visible externally; for example,
-         * you will not be sending network packets continuously instead of
-         * every 100ms.
          */
-        seqlock_write_lock(&timers_state.vm_clock_seqlock);
-        if (vm_clock_warp_start == -1 || vm_clock_warp_start > clock) {
-            vm_clock_warp_start = clock;
+        if (!icount_sleep) {
+            /*
+             * We never let VCPUs sleep in no sleep icount mode.
+             * If there is a pending QEMU_CLOCK_VIRTUAL timer we just advance
+             * to the next QEMU_CLOCK_VIRTUAL event and notify it.
+             * It is useful when we want a deterministic execution time,
+             * isolated from host latencies.
+             */
+            seqlock_write_lock(&timers_state.vm_clock_seqlock);
+            timers_state.qemu_icount_bias += deadline;
+            seqlock_write_unlock(&timers_state.vm_clock_seqlock);
+            qemu_clock_notify(QEMU_CLOCK_VIRTUAL);
+        } else {
+            /*
+             * We do stop VCPUs and only advance QEMU_CLOCK_VIRTUAL after some
+             * "real" time, (related to the time left until the next event) has
+             * passed. The QEMU_CLOCK_VIRTUAL_RT clock will do this.
+             * This avoids that the warps are visible externally; for example,
+             * you will not be sending network packets continuously instead of
+             * every 100ms.
+             */
+            seqlock_write_lock(&timers_state.vm_clock_seqlock);
+            if (vm_clock_warp_start == -1 || vm_clock_warp_start > clock) {
+                vm_clock_warp_start = clock;
+            }
+            seqlock_write_unlock(&timers_state.vm_clock_seqlock);
+            timer_mod_anticipate(icount_warp_timer, clock + deadline);
         }
-        seqlock_write_unlock(&timers_state.vm_clock_seqlock);
-        timer_mod_anticipate(icount_warp_timer, clock + deadline);
     } else if (deadline == 0) {
         qemu_clock_notify(QEMU_CLOCK_VIRTUAL);
     }
@@ -504,9 +520,11 @@ void configure_icount(QemuOpts *opts, Error **errp)
         }
         return;
     }
+    if (icount_sleep) {
+        icount_warp_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL_RT,
+                                         icount_warp_rt, NULL);
+    }
     icount_align_option = qemu_opt_get_bool(opts, "align", false);
-    icount_warp_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL_RT,
-                                     icount_warp_rt, NULL);
     if (strcmp(option, "auto") != 0) {
         errno = 0;
         icount_time_shift = strtol(option, &rem_str, 0);
-- 
2.4.2