docs: Example usage of pvrdtscp algorithm

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>

1 files changed, 307 insertions, 0 deletions

diff --git a/docs/misc/pvrdtscp.c b/docs/misc/pvrdtscp.c
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+/* pvrdtscp algorithm
+ *
+ * This sample code demonstrates the use of the paravirtualized rdtscp
+ * algorithm.  Using this algorithm, an application may communicate with
+ * the Xen hypervisor (version 4.0+) to obtain timestamp information which
+ * is both monotonically increasing and has a fixed 1 GHz rate, even across
+ * migrations between machines with different TSC rates and offsets.
+ * Further,the algorithm provides performance near the performance of a
+ * native rdtsc/rdtscp instruction -- much faster than emulation PROVIDED
+ * the application is running on a machine on which the rdtscp instruction
+ * is supported and TSC is "safe". The application must also be running in a
+ * PV domain.  (HVM domains may be supported at a later time.) On machines
+ * where TSC is unsafe or the rdtscp instruction is not supported, Xen
+ * (v4.0+) provides emulation which is slower but consistent with the pvrdtscp
+ * algorithm, thus providing support for the algorithm for live migration
+ * across all machines.
+ *
+ * More information can be found within the Xen (4.0+) source tree at
+ *  docs/misc/tscmode.txt
+ *
+ * Copyright (c) 2009 Oracle Corporation and/or its affiliates.
+ * All rights reserved
+ * Written by: Dan Magenheimer <dan.magenheimer@oracle.com>
+ * 
+ * This code is derived from code licensed under the GNU
+ * General Public License ("GPL") version 2 and is therefore itself
+ * also licensed under the GPL version 2.
+ *
+ * This code is known to compile and run on Oracle Enterprise Linux 5 Update 2
+ * using gcc version 4.1.2, but its purpose is to describe the pvrdtscp
+ * algorithm and its ABI to Xen version 4.0+ 
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/wait.h>
+
+#ifdef __LP64__
+#define __X86_64__
+typedef unsigned short u16;
+typedef unsigned int u32;
+typedef unsigned long u64;
+typedef int i32;
+typedef long i64;
+#define NSEC_PER_SEC 1000000000
+#else
+#define __X86_32__
+typedef unsigned int u16;
+typedef unsigned long u32;
+typedef unsigned long long u64;
+typedef long i32;
+typedef long long i64;
+#define NSEC_PER_SEC 1000000000L
+#endif
+
+static inline void hvm_cpuid(u32 idx, u32 sub,
+				u32 *eax, u32 *ebx, u32 *ecx, u32 *edx)
+{
+	*eax = idx, *ecx = sub;
+	asm("cpuid" : "=a" (*eax), "=b" (*ebx), "=c" (*ecx), "=d" (*edx)
+	    : "0" (*eax), "2" (*ecx));
+}
+
+static inline void pv_cpuid(u32 idx, u32 sub,
+				u32 *eax, u32 *ebx, u32 *ecx, u32 *edx)
+{
+	*eax = idx, *ecx = sub;
+	asm volatile ( "ud2a ; .ascii \"xen\"; cpuid" : "=a" (*eax),
+            "=b" (*ebx), "=c" (*ecx), "=d" (*edx) : "0" (*eax), "2" (*ecx));
+}
+
+static inline u64 do_rdtscp(u32 *aux)
+{
+static u64 last = 0;
+	u32 lo32, hi32;
+	u64 val;
+
+	asm volatile(".byte 0x0f,0x01,0xf9":"=a"(lo32),"=d"(hi32),"=c" (*aux));
+	val = lo32 | ((u64)hi32 << 32);
+	return val;
+}
+
+static inline int get_xen_tsc_mode(void)
+{
+	u32 val, dummy1, dummy2, dummy3;
+	pv_cpuid(0x40000003,0,&dummy1,&val,&dummy2,&dummy3);
+	return val;
+}
+
+static inline int get_xen_vtsc(void)
+{
+	u32 val, dummy1, dummy2, dummy3;
+	pv_cpuid(0x40000003,0,&val,&dummy1,&dummy2,&dummy3);
+	return val & 1;
+}
+
+static inline int get_xen_vtsc_khz(void)
+{
+	u32 val, dummy1, dummy2, dummy3;
+	pv_cpuid(0x40000003,0,&dummy1,&dummy2,&val,&dummy3);
+	return val;
+}
+
+static inline u32 get_xen_cpu_khz(void)
+{
+	u32 cpu_khz, dummy1, dummy2, dummy3;
+	pv_cpuid(0x40000003,2,&cpu_khz,&dummy1,&dummy2,&dummy3);
+	return cpu_khz;
+}
+
+static inline u32 get_xen_incarnation(void)
+{
+	u32 incarn, dummy1, dummy2, dummy3;
+	pv_cpuid(0x40000003,0,&dummy1,&dummy2,&dummy3,&incarn);
+	return incarn;
+}
+
+static inline void get_xen_time_values(u64 *offset, u32 *mul_frac, u32 *shift)
+{
+	u32 off_lo, off_hi, sys_lo, sys_hi, dummy;
+
+	pv_cpuid(0x40000003,1,&off_lo,&off_hi,mul_frac,shift);
+	*offset = off_lo | ((u64)off_hi << 32);
+}
+
+static inline u64 scale_delta(u64 delta, u32 tsc_mul_frac, i32 tsc_shift)
+{
+    u64 product;
+#ifdef __X86_32__
+    u32 tmp1, tmp2;
+#endif
+
+    if ( tsc_shift < 0 )
+        delta >>= -tsc_shift;
+    else
+        delta <<= tsc_shift;
+
+#ifdef __X86_32__
+    asm (
+        "mul  %5       ; "
+        "mov  %4,%%eax ; "
+        "mov  %%edx,%4 ; "
+        "mul  %5       ; "
+        "xor  %5,%5    ; "
+        "add  %4,%%eax ; "
+        "adc  %5,%%edx ; "
+        : "=A" (product), "=r" (tmp1), "=r" (tmp2)
+        : "a" ((u32)delta), "1" ((u32)(delta >> 32)), "2" (tsc_mul_frac) );
+#else
+    asm (
+        "mul %%rdx ; shrd $32,%%rdx,%%rax"
+        : "=a" (product) : "0" (delta), "d" ((u64)tsc_mul_frac) );
+#endif
+
+    return product;
+}
+
+static inline u64 get_pvrdtscp_timestamp(int *discontinuity)
+{
+	static int firsttime = 1;
+	static u64 last_pvrdtscp_timestamp = 0;
+	static u32 last_tsc_aux;
+	static u64 xen_ns_offset;
+	static u32 xen_tsc_to_ns_mul_frac, xen_tsc_to_ns_shift;
+	u32 this_tsc_aux;
+	u64 timestamp, cur_tsc, cur_ns;
+
+	if (firsttime) {
+		cur_tsc = do_rdtscp(&last_tsc_aux);
+		get_xen_time_values(&xen_ns_offset, &xen_tsc_to_ns_mul_frac,
+					&xen_tsc_to_ns_shift);
+		cur_ns = scale_delta(cur_tsc, xen_tsc_to_ns_mul_frac,
+					xen_tsc_to_ns_shift);
+		timestamp = cur_ns - xen_ns_offset;
+		last_pvrdtscp_timestamp = timestamp;
+		firsttime = 0;
+	}
+	cur_tsc = do_rdtscp(&this_tsc_aux);
+	*discontinuity = 0;
+	while (this_tsc_aux != last_tsc_aux) {
+		/* if tsc_aux changed, try again */
+		last_tsc_aux = this_tsc_aux;
+		get_xen_time_values(&xen_ns_offset, &xen_tsc_to_ns_mul_frac,
+					&xen_tsc_to_ns_shift);
+		cur_tsc = do_rdtscp(&this_tsc_aux);
+		*discontinuity = 1;
+	}
+
+	/* compute nsec from TSC and Xen time values */
+	cur_ns = scale_delta(cur_tsc, xen_tsc_to_ns_mul_frac,
+					xen_tsc_to_ns_shift);
+	timestamp = cur_ns - xen_ns_offset;
+
+	/* enforce monotonicity just in case */
+	if ((i64)(timestamp - last_pvrdtscp_timestamp) > 0)
+		last_pvrdtscp_timestamp = timestamp;
+	else {
+		/* this should never happen but we'll check it anyway in
+		 * case of some strange combination of scaling errors
+		 * occurs across a very fast migration */
+		printf("Time went backwards by %lluns\n",
+		    (unsigned long long)(last_pvrdtscp_timestamp-timestamp));
+		timestamp = ++last_pvrdtscp_timestamp;
+	}
+	return timestamp;
+}
+
+#define HVM 1
+#define PVM 0
+
+static int running_on_xen(int hvm, u16 *version_major, u16 *version_minor)
+{
+	u32 eax, ebx, ecx, edx, base;
+	union { char csig[16]; u32 u[4]; } sig;
+
+	for (base=0x40000000; base < 0x40010000; base += 0x100) {
+		if (hvm==HVM)
+			hvm_cpuid(base,0,&eax,&ebx,&ecx,&edx);
+		else
+			pv_cpuid(base,0,&eax,&ebx,&ecx,&edx);
+		sig.u[0] = ebx; sig.u[1] = ecx; sig.u[2] = edx;
+		sig.csig[12] = '\0';
+		if (!strcmp("XenVMMXenVMM",&sig.csig[0]) && (eax >= (base+2))) {
+				if (hvm==HVM)
+					hvm_cpuid(base+1,0,&eax,&ebx,&ecx,&edx);
+				else
+					pv_cpuid(base+1,0,&eax,&ebx,&ecx,&edx);
+				*version_major = (eax >> 16) & 0xffff;
+				*version_minor = eax & 0xffff;
+				return 1;
+		}
+	}
+	return 0;
+}
+
+main(int ac, char **av)
+{
+	u32 dummy;
+	u16 version_hi, version_lo;
+	u64 ts, last_ts;
+	int status, discontinuity = 0;
+	pid_t pid;
+
+	if (running_on_xen(HVM,&version_hi,&version_lo)) {
+		printf("running on Xen v%d.%d as an HVM domain, "
+			"pvrdtsc not supported, exiting\n",
+			(int)version_hi, (int)version_lo);
+		exit(0);
+	}
+	pid = fork();
+	if (pid == -1) {
+		fprintf(stderr,"Huh? Fork failed\n");
+		return 0;
+	}
+	else if (pid == 0) { /* child */
+		pv_cpuid(0x40000000,0,&dummy,&dummy,&dummy,&dummy);
+		exit(0);
+	}
+	waitpid(pid,&status,0);
+	if (!WIFEXITED(status))
+		exit(0);
+	if (!running_on_xen(PVM,&version_hi,&version_lo)) {
+		printf("not running on Xen, exiting\n");
+		exit(0);
+	}
+	printf("running on Xen v%d.%d as a PV domain\n",
+		(int)version_hi, (int)version_lo);
+	if ( version_hi <= 3 ) {
+		printf("pvrdtscp requires Xen version 4.0 or greater\n");
+		/* exit(0); FIXME after xen-unstable is officially v4.0 */
+	}
+	if ( get_xen_tsc_mode() != 3 )
+		printf("tsc_mode not pvrdtscp, set tsc_mode=3, exiting\n");
+
+	/* OK, we are on Xen, now loop forever checking timestamps */
+	ts = get_pvrdtscp_timestamp(&discontinuity);
+	printf("Starting with ts=%lluns 0x%llx (%llusec)\n",ts,ts,ts/NSEC_PER_SEC);
+	printf("incarn=%d: vtsc=%d, vtsc_khz=%lu, phys cpu_khz=%lu\n",
+				(unsigned long)get_xen_incarnation(),
+				(unsigned long)get_xen_vtsc(),
+				(unsigned long)get_xen_vtsc_khz(),
+				(unsigned long)get_xen_cpu_khz());
+	ts = get_pvrdtscp_timestamp(&discontinuity);
+	last_ts = ts;
+	while (1) {
+		ts = get_pvrdtscp_timestamp(&discontinuity);
+		if (discontinuity)
+			printf("migrated/restored, incarn=%d: "
+                               "vtsc now %d, vtsc_khz=%lu, phys cpu_khz=%lu\n",
+				(unsigned long)get_xen_incarnation(),
+				(unsigned long)get_xen_vtsc(),
+				(unsigned long)get_xen_vtsc_khz(),
+				(unsigned long)get_xen_cpu_khz());
+		if (ts < last_ts)
+			/* this should NEVER happen, especially since there
+			 * is a check for it in get_pvrdtscp_timestamp() */
+			printf("Time went backwards: %lluns (%llusec)\n",
+				last_ts-ts,(last_ts-ts)/NSEC_PER_SEC);
+		if (ts > last_ts + 200000000LL)
+			/* this is OK, usually about 2sec for save/restore
+			 * and a fraction of a second for live migrate */
+			printf("Time jumped forward %lluns (%llusec)\n",
+				ts-last_ts,(ts-last_ts)/NSEC_PER_SEC);
+		last_ts = ts;
+	}
+}