From 998e1ef11f9c9c86010fa31273e071a436a5686d Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Tue, 29 Mar 2016 08:44:56 +0000
Subject: [PATCH] Calibrate the frequency of the of the native_lapic_ipi_wait()
 loop, and avoid a delay while waiting for IPI delivery acknowledgement in
 xAPIC mode.  This makes the loop exit immediately after the delivery bit in
 APIC_ICR register is set, instead of waiting for some microseconds.

We only need to ensure that some amount of time is allowed for the
LAPIC to react to the command, and we need that the wait time is
finite and reasonable.  For that reasons, it is irrelevant if the CPU
frequency or throttling decrease the speed and make the loop,
calibrated for full CPU speed at boot time, execute somewhat slower.

Discussed with:	bde, jhb
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
---
 sys/x86/x86/local_apic.c | 55 +++++++++++++++++++++++++++++-----------
 1 file changed, 40 insertions(+), 15 deletions(-)

diff --git a/sys/x86/x86/local_apic.c b/sys/x86/x86/local_apic.c
index d511ba811604..85d80c2e83bb 100644
--- a/sys/x86/x86/local_apic.c
+++ b/sys/x86/x86/local_apic.c
@@ -172,6 +172,7 @@ int lapic_eoi_suppression;
 static int lapic_timer_tsc_deadline;
 static u_long lapic_timer_divisor;
 static struct eventtimer lapic_et;
+static uint64_t lapic_ipi_wait_mult;
 
 SYSCTL_NODE(_hw, OID_AUTO, apic, CTLFLAG_RD, 0, "APIC options");
 SYSCTL_INT(_hw_apic, OID_AUTO, x2apic_mode, CTLFLAG_RD, &x2apic_mode, 0, "");
@@ -403,6 +404,7 @@ lvt_mode(struct lapic *la, u_int pin, uint32_t value)
 static void
 native_lapic_init(vm_paddr_t addr)
 {
+	uint64_t r;
 	uint32_t ver;
 	u_int regs[4];
 	int i, arat;
@@ -503,6 +505,34 @@ native_lapic_init(vm_paddr_t addr)
 		TUNABLE_INT_FETCH("hw.lapic_eoi_suppression",
 		    &lapic_eoi_suppression);
 	}
+
+#define LOOPS 1000000
+	/*
+	 * Calibrate the busy loop waiting for IPI ack in xAPIC mode.
+	 * lapic_ipi_wait_mult contains the number of iterations which
+	 * approximately delay execution for 1 microsecond (the
+	 * argument to native_lapic_ipi_wait() is in microseconds).
+	 *
+	 * We assume that TSC is present and already measured.
+	 * Possible TSC frequency jumps are irrelevant to the
+	 * calibration loop below, the CPU clock management code is
+	 * not yet started, and we do not enter sleep states.
+	 */
+	KASSERT((cpu_feature & CPUID_TSC) != 0 && tsc_freq != 0,
+	    ("TSC not initialized"));
+	r = rdtsc();
+	for (r = 0; r < LOOPS; r++) {
+		(void)lapic_read_icr_lo();
+		ia32_pause();
+	}
+	r = rdtsc() - r;
+	lapic_ipi_wait_mult = (r * 1000000) / tsc_freq / LOOPS;
+	if (bootverbose) {
+		printf("LAPIC: ipi_wait() us multiplier %jd (r %jd tsc %jd)\n",
+		    (uintmax_t)lapic_ipi_wait_mult, (uintmax_t)r,
+		    (uintmax_t)tsc_freq);
+	}
+#undef LOOPS
 }
 
 /*
@@ -1716,31 +1746,26 @@ SYSINIT(apic_setup_io, SI_SUB_INTR, SI_ORDER_THIRD, apic_setup_io, NULL);
  * private to the MD code.  The public interface for the rest of the
  * kernel is defined in mp_machdep.c.
  */
+
+/*
+ * Wait delay microseconds for IPI to be sent.  If delay is -1, we
+ * wait forever.
+ */
 static int
 native_lapic_ipi_wait(int delay)
 {
-	int x;
+	uint64_t i, counter;
 
 	/* LAPIC_ICR.APIC_DELSTAT_MASK is undefined in x2APIC mode */
-	if (x2apic_mode)
+	if (x2apic_mode || delay == -1)
 		return (1);
 
-	/*
-	 * Wait delay microseconds for IPI to be sent.  If delay is
-	 * -1, we wait forever.
-	 */
-	if (delay == -1) {
-		while ((lapic_read_icr_lo() & APIC_DELSTAT_MASK) !=
-		    APIC_DELSTAT_IDLE)
-			ia32_pause();
-		return (1);
-	}
-
-	for (x = 0; x < delay; x++) {
+	counter = lapic_ipi_wait_mult * delay;
+	for (i = 0; i < counter; i++) {
 		if ((lapic_read_icr_lo() & APIC_DELSTAT_MASK) ==
 		    APIC_DELSTAT_IDLE)
 			return (1);
-		DELAY(1);
+		ia32_pause();
 	}
 	return (0);
 }