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Index: sys/kern/subr_smp.c
===================================================================
--- sys/kern/subr_smp.c
+++ sys/kern/subr_smp.c
@@ -56,7 +56,8 @@
MALLOC_DEFINE(M_TOPO, "toponodes", "SMP topology data");
volatile cpuset_t stopped_cpus;
-volatile cpuset_t started_cpus;
+/* Align to attempt to keep started_cpus within a cache line for MWAIT. */
+volatile cpuset_t started_cpus __aligned(CACHE_LINE_SIZE);
volatile cpuset_t suspended_cpus;
cpuset_t hlt_cpus_mask;
cpuset_t logical_cpus_mask;
Index: sys/x86/x86/mp_x86.c
===================================================================
--- sys/x86/x86/mp_x86.c
+++ sys/x86/x86/mp_x86.c
@@ -161,6 +161,10 @@
unsigned int boot_address;
+static bool stop_mwait = false;
+SYSCTL_BOOL(_machdep, OID_AUTO, stop_mwait, CTLFLAG_RWTUN, &stop_mwait, 0,
+ "Use MONITOR/MWAIT when stopping CPU");
+
#define MiB(v) (v ## ULL << 20)
void
@@ -1389,6 +1393,17 @@
cpustop_handler_post(cpu);
}
+static bool
+range_can_be_monitored(volatile void *start, size_t length)
+{
+ uint64_t addr, preceding;
+
+ addr = (uintptr_t)start;
+ preceding = addr & (cpu_mon_min_size - 1);
+
+ return (preceding + length <= cpu_mon_min_size);
+}
+
/*
* Handle an IPI_STOP by saving our current context and spinning until we
* are resumed.
@@ -1397,6 +1412,7 @@
cpustop_handler(void)
{
u_int cpu;
+ bool use_mwait;
cpu = PCPU_GET(cpuid);
@@ -1405,8 +1421,18 @@
/* Indicate that we are stopped */
CPU_SET_ATOMIC(cpu, &stopped_cpus);
+ use_mwait = (stop_mwait && (cpu_feature2 & CPUID2_MON) &&
+ range_can_be_monitored(&started_cpus, sizeof(started_cpus)));
+
/* Wait for restart */
while (!CPU_ISSET(cpu, &started_cpus)) {
+ if (use_mwait) {
+ cpu_monitor(__DEVOLATILE(void *, &started_cpus), 0, 0);
+ if (!CPU_ISSET(cpu, &started_cpus))
+ cpu_mwait(0, MWAIT_C1);
+ continue;
+ }
+
ia32_pause();
/*

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