'\" t
.\"     Title: libtraceevent
.\"    Author: [FIXME: author] [see http://www.docbook.org/tdg5/en/html/author]
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\"      Date: 01/14/2024
.\"    Manual: libtraceevent Manual
.\"    Source: libtraceevent 1.8.2
.\"  Language: English
.\"
.TH "LIBTRACEEVENT" "3" "01/14/2024" "libtraceevent 1\&.8\&.2" "libtraceevent Manual"
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.nh
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.\" * MAIN CONTENT STARTS HERE *
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.SH "NAME"
tep_plugin_kvm_get_func, tep_plugin_kvm_put_func \- Add function name for instruction pointer of kvm plugin
.SH "SYNOPSIS"
.sp
.nf
\fB#include <event\-parse\&.h>\fR

const char *\fBtep_plugin_kvm_get_func\fR(struct tep_event *event,
                                    struct tep_record *record,
                                    unsigned long long *paddr);
void \fBtep_plugin_kvm_put_func\fR(const char *func);
.fi
.SH "DESCRIPTION"
.sp
The functions \fBtep_plugin_kvm_get_func()\fR and \fBtep_plugin_kvm_put_func()\fR are not to be called by an application, but instead are to be defined by an application\&.
.sp
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\&.
.sp
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\&.
.sp
The KVM plugin for libtraceevent is called on these events, and then calls \fBtep_plugin_kvm_get_func()\fR 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\&.
.sp
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 \fB\-rdynamic\fR\&. 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)\&.
.sp
If the application defines \fBtep_plugin_kvm_get_func()\fR, it must use the above prototype\&. The \fIevent\fR will hold the KVM event that has the instruction pointer field\&. The \fIrecord\fR will be the instance of that event\&. The application\(cqs function does not need to use these parameters, but they may be useful for finding the function name for the address\&. The \fIpaddr\fR 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 \fIpaddr\fR to that address, and the KVM plugin will also append a "+offset" to the function name where the offset is the original value in \fIpaddr\fR minus the value in \fIpaddr\fR when it is called\&. Finally, the application should return the function name as a nul terminated string if one is found\&.
.sp
If the returned string of \fBtep_plugin_kvm_get_func()\fR was allocated, the KVM plugin will call \fBtep_plugin_kvm_put_func()\fR when it is through with it, passing the value returned by \fBtep_plugin_kvm_get_func()\fR as \fIfunc\fR\&. This allows the application to free it if necessary\&.
.SH "RETURN VALUE"
.sp
The \fBtep_plugin_kvm_get_func()\fR 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 \fIpaddr\fR\&. It can then optionally update the \fIpaddr\fR to a value that holds the start of the function\&. The string returned may be freed by the \fBtep_plugin_kvm_put_func()\fR that the application should define to clean up the string\&.
.sp
The below example needs to be compiled with the \fB\-rdynamic\fR flag so that the dynamic linker can resolve the \fBtep_plugin_kvm_get_func()\fR and \fBtep_plugin_kvm_put_func()\fR functions\&.
.sp
When run against a trace\&.dat file produced by \fBtrace\-cmd(1)\fR recording the kvm_exit and kvm_entry events on a guest, and then the guest\(cqs /proc/kallsyms file is passed as the second parameter, the output produced will look something like:
.sp
.if n \{\
.RS 4
.\}
.nf
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
.fi
.if n \{\
.RE
.\}
.sp
But without those callbacks, it would look like:
.sp
.if n \{\
.RS 4
.\}
.nf
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
.fi
.if n \{\
.RE
.\}
.SH "EXAMPLE"
.sp
.if n \{\
.RS 4
.\}
.nf
#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\et%6\&.1000d [%03d] %s\et%s\en",
                        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\en");
                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] = \*(Aq\e0\*(Aq;

        if (tep_parse_kallsyms(guest_tep, buf) < 0) {
                printf("Failed to parse %s\en", 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);
}
.fi
.if n \{\
.RE
.\}
.SH "FILES"
.sp
.if n \{\
.RS 4
.\}
.nf
\fBevent\-parse\&.h\fR
        Header file to include in order to have access to the library APIs\&.
\fB\-ltraceevent\fR
        Linker switch to add when building a program that uses the library\&.
.fi
.if n \{\
.RE
.\}
.SH "SEE ALSO"
.sp
\fBlibtraceevent\fR(3), \fBtrace\-cmd\fR(1)
.SH "REPORTING BUGS"
.sp
Report bugs to <\m[blue]\fBlinux\-trace\-devel@vger\&.kernel\&.org\fR\m[]\&\s-2\u[1]\d\s+2>
.SH "LICENSE"
.sp
libtraceevent is Free Software licensed under the GNU LGPL 2\&.1
.SH "RESOURCES"
.sp
\m[blue]\fBhttps://git\&.kernel\&.org/pub/scm/libs/libtrace/libtraceevent\&.git/\fR\m[]
.SH "NOTES"
.IP " 1." 4
linux-trace-devel@vger.kernel.org
.RS 4
\%mailto:linux-trace-devel@vger.kernel.org
.RE