migration/postcopy: Cleanup the total blocktime accounting

The variable vcpu_total_blocktime isn't easy to follow.  In reality, it
wants to capture the case where all vCPUs are stopped, and now there will
be some vCPUs starts running.

The name now starts to conflict with vcpu_blocktime_total[], meanwhile it's
actually not necessary to have the variable at all: since nobody is
touching smp_cpus_down except ourselves, we can safely do the calculation
at the end before decrementing smp_cpus_down.

Hopefully this makes the logic easier to read, side benefit is we drop one
temp var.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250613141217.474825-12-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/postcopy-ram.c

@@ -972,7 +972,6 @@ static void mark_postcopy_blocktime_end(uintptr_t addr)
     MachineState *ms = MACHINE(qdev_get_machine());
     unsigned int smp_cpus = ms->smp.cpus;
     int i, affected_cpu = 0;
-    bool vcpu_total_blocktime = false;
     uint64_t read_vcpu_time, current;
 
     if (!dc) {
@@ -994,20 +993,19 @@ static void mark_postcopy_blocktime_end(uintptr_t addr)
         dc->vcpu_addr[i] = 0;
         vcpu_blocktime = current - read_vcpu_time;
         affected_cpu += 1;
-        /* we need to know is that mark_postcopy_end was due to
-         * faulted page, another possible case it's prefetched
-         * page and in that case we shouldn't be here */
-        if (!vcpu_total_blocktime && dc->smp_cpus_down == smp_cpus) {
-            vcpu_total_blocktime = true;
-        }
         /* continue cycle, due to one page could affect several vCPUs */
         dc->vcpu_blocktime_total[i] += vcpu_blocktime;
     }
 
-    dc->smp_cpus_down -= affected_cpu;
-    if (vcpu_total_blocktime) {
+    /*
+     * If all vCPUs used to be down, and copying this page would free some
+     * vCPUs, then the system-level blocktime ends here.
+     */
+    if (dc->smp_cpus_down == smp_cpus && affected_cpu) {
         dc->total_blocktime += current - dc->last_begin;
     }
+    dc->smp_cpus_down -= affected_cpu;
+
     trace_mark_postcopy_blocktime_end(addr, dc->total_blocktime,
                                       affected_cpu);
 }