Dan Magenheimer
2009-Dec-09 16:31 UTC
[Xen-devel] [PATCH] tscmode: documentation (in C code) for pvrdtscp algorithm
This code describes the pvrdtscp algorithm and the new
ABI between Xen and applications.
Keir, I''m no sure where to put this... though it is a
C program, it is essentially documentation. So, I''ve
just attached as a file, not as a patch.
Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
/* 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;
}
}
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