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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | EXAMPLE | FILES | SEE ALSO | REPORTING BUGS | LICENSE | RESOURCES | NOTES | COLOPHON |
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LIBTRACEEVENT(3) libtraceevent Manual LIBTRACEEVENT(3)
tep_plugin_kvm_get_func, tep_plugin_kvm_put_func - Add function
name for instruction pointer of kvm plugin
#include <event-parse.h>
const char *tep_plugin_kvm_get_func(struct tep_event *event,
struct tep_record *record,
unsigned long long *paddr);
void tep_plugin_kvm_put_func(const char *func);
The functions tep_plugin_kvm_get_func() and
tep_plugin_kvm_put_func() are not to be called by an application,
but instead are to be defined by an application.
Certain events (like kvm_exit and kvm_entry) have the instruction
pointer of where in the guest the context changed from guest to
host. As the host only knows the instruction pointer and does not
have information about what function in the guest that
instruction pointer belongs to, it can only print the address.
But the application may have more information about the guest,
and know where the guest was when the exit occurred, and also
even know the function name of that address.
The KVM plugin for libtraceevent is called on these events, and
then calls tep_plugin_kvm_get_func() to see if that function can
resolve the instruction pointer address to a real function name.
If the return is non NULL, it will print the function in the
output for that event.
These functions are currently defined as weak functions within
the plugin, as to not require them to be defined elsewhere. For
an application to override the weak function, it will need to
define the function in a file that gets compiled with -rdynamic.
That will tell the dynamic linker to examine that object file and
use function names to resolve weak functions in other shared
objects (in this case the KVM plugin shared object).
If the application defines tep_plugin_kvm_get_func(), it must use
the above prototype. The event will hold the KVM event that has
the instruction pointer field. The record will be the instance of
that event. The application’s function does not need to use these
parameters, but they may be useful for finding the function name
for the address. The paddr is a pointer to a 64 bit value (where
only 32 bits may be used on 32 bit machines). This value is the
instruction pointer to look up. If the application knows the
start address of the function as well, it can set paddr to that
address, and the KVM plugin will also append a "+offset" to the
function name where the offset is the original value in paddr
minus the value in paddr when it is called. Finally, the
application should return the function name as a nul terminated
string if one is found.
If the returned string of tep_plugin_kvm_get_func() was
allocated, the KVM plugin will call tep_plugin_kvm_put_func()
when it is through with it, passing the value returned by
tep_plugin_kvm_get_func() as func. This allows the application to
free it if necessary.
The tep_plugin_kvm_get_func() is not to be called by the
application but instead is to be defined by the application. It
should return a nul terminated string representing the function
for the given instruction pointer passed to it by reference in
paddr. It can then optionally update the paddr to a value that
holds the start of the function. The string returned may be freed
by the tep_plugin_kvm_put_func() that the application should
define to clean up the string.
The below example needs to be compiled with the -rdynamic flag so
that the dynamic linker can resolve the tep_plugin_kvm_get_func()
and tep_plugin_kvm_put_func() functions.
When run against a trace.dat file produced by trace-cmd(1)
recording the kvm_exit and kvm_entry events on a guest, and then
the guest’s /proc/kallsyms file is passed as the second
parameter, the output produced will look something like:
CPU 0/KVM-20407 83156.177626 [000] kvm_exit reason APIC_ACCESS rip 0xffffffffb0056ee2 exit native_apic_mem_write+0x2 info 10b0 0
CPU 0/KVM-20407 83156.177632 [000] kvm_entry vcpu 0 rip 0xffffffffb0056ee8 enter native_apic_mem_write+0x8
But without those callbacks, it would look like:
CPU 0/KVM-20407 83156.177626 [000] kvm_exit reason APIC_ACCESS rip 0xffffffffb0056ee2 info 10b0 0
CPU 0/KVM-20407 83156.177632 [000] kvm_entry vcpu 0 rip 0xffffffffb0056ee8
#include <stdio.h>
#include <stdlib.h>
#include <event-parse.h>
#include <trace-cmd.h>
#include <sys/stat.h>
static struct tep_handle *tep;
const char *tep_plugin_kvm_get_func(struct tep_event *event, struct tep_record *record,
unsigned long long *paddr)
{
const char *func;
char *event_func;
char *ename;
func = tep_find_function(tep, *paddr);
if (!func)
return NULL;
if (strcmp(event->name, "kvm_exit") == 0)
ename = "exit";
else
ename = "enter";
/*
* Normally, passing back func directly is sufficient and then
* tep_plugin_kvm_put_func() would not be required. But this example
* is showing how to handle allocation of the returned string.
*/
event_func = malloc(strlen(ename) + strlen(func) + 2);
if (!event_func)
return NULL;
sprintf(event_func, "%s %s", ename, func);
*paddr = tep_find_function_address(tep, *paddr);
return event_func;
}
void tep_plugin_kvm_put_func(const char *func)
{
char *f = (char *)func;
free(f);
}
static int show_event(struct tracecmd_input *handle, struct tep_event *event,
struct tep_record *record, int cpu, void *data)
{
static struct trace_seq seq;
tep = data;
if (!seq.buffer)
trace_seq_init(&seq);
trace_seq_reset(&seq);
tep_print_event(tracecmd_get_tep(handle), &seq, record,
"%s-%d\t%6.1000d [%03d] %s\t%s\n",
TEP_PRINT_COMM, TEP_PRINT_PID,
TEP_PRINT_TIME, TEP_PRINT_CPU,
TEP_PRINT_NAME, TEP_PRINT_INFO);
trace_seq_terminate(&seq);
trace_seq_do_printf(&seq);
return 0;
}
int main(int argc, char **argv)
{
struct tracecmd_input *handle;
struct tep_handle *guest_tep;
struct stat st;
FILE *fp;
char *buf;
if (argc < 3) {
printf("usage: trace.dat guest_kallsyms_file\n");
exit(-1);
}
handle = tracecmd_open(argv[1], 0);
if (!handle) {
perror(argv[1]);
exit(-1);
}
/* Just for kallsyms parsing */
guest_tep = tep_alloc();
if (!guest_tep)
exit(-1);
if (stat(argv[2], &st) < 0) {
perror(argv[2]);
exit(-1);
}
buf = malloc(st.st_size + 1);
if (!buf)
exit(-1);
fp = fopen(argv[2], "r");
if (!fp) {
perror(argv[2]);
exit(-1);
}
if (fread(buf, st.st_size, 1, fp) < 0) {
perror(argv[2]);
exit(-1);
}
buf[st.st_size] = '\0';
if (tep_parse_kallsyms(guest_tep, buf) < 0) {
printf("Failed to parse %s\n", argv[2]);
exit(-1);
}
free(buf);
tracecmd_follow_event(handle, "kvm", "kvm_exit", show_event, guest_tep);
tracecmd_follow_event(handle, "kvm", "kvm_entry", show_event, guest_tep);
tracecmd_iterate_events(handle, NULL, 0, NULL, NULL);
tep_free(guest_tep);
tracecmd_close(handle);
}
event-parse.h
Header file to include in order to have access to the library APIs.
-ltraceevent
Linker switch to add when building a program that uses the library.
libtraceevent(3), trace-cmd(1)
Report bugs to <linux-trace-devel@vger.kernel.org[1]>
libtraceevent is Free Software licensed under the GNU LGPL 2.1
https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/
1. linux-trace-devel@vger.kernel.org
mailto:linux-trace-devel@vger.kernel.org
This page is part of the libtraceevent (Linux kernel trace event
library) project. Information about the project can be found at
⟨https://www.trace-cmd.org/⟩. If you have a bug report for this
manual page, see ⟨https://www.trace-cmd.org/⟩. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git⟩
on 2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-05-17.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
libtraceevent 1.7.3 09/24/2023 LIBTRACEEVENT(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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