Hi,
While attempting to use "perf kvm --guestkallsyms=g-kallsyms report",
I have found that perf 5.13-rc2 fails to convert IP values to symbols.
After spending a while debugging, what I've found is:
1) perf_session__new() will not read the gust kernel symbols in this
case (data is non-NULL, and is open read-only.) The comments say
that symbol reading will be deferred to the MMAP and MMAP2 events.
2) all of the MMAP and MMAP2 events seem to be for
PERF_RECORD_MISC_KERNEL and PERF_RECORD_MISC_USER. There are none
for PERF_RECORD_MISC_GUEST_KERNEL.
I have found that disabling the checks in perf_session__new() mentioned
in (1) allows the command above to give me symbolic information. As I
don't know the code all that well, I'm not sure if this is a correct
fix, or whether the problem lies in the "perf kvm record" side - that
depends whether there are supposed to be MMAP/MMAP2 events for the
guest kernel.
Below is the debug diff I've been using so far... (building out of the
kernel tree.) The change to tools/perf/util/session.c seems to be what
has fixed the problem for me, but I don't think this is the correct
fix.
diff -ur -x '*.o' -x '*.cmd' perf-5.13.0-rc2.orig/tools/perf/util/machine.c perf-5.13.0-rc2/tools/perf/util/machine.c
--- perf-5.13.0-rc2.orig/tools/perf/util/machine.c 2021-05-19 15:04:47.000000000 +0100
+++ perf-5.13.0-rc2/tools/perf/util/machine.c 2021-05-20 14:41:55.425771482 +0100
@@ -1234,7 +1234,7 @@
char path[PATH_MAX];
pid_t pid;
char *endp;
-
+printf("machines__create_guest_kernel_maps: %s\n", symbol_conf.default_guest_kallsyms);
if (symbol_conf.default_guest_vmlinux_name ||
symbol_conf.default_guest_modules ||
symbol_conf.default_guest_kallsyms) {
@@ -1293,7 +1293,7 @@
int machines__create_kernel_maps(struct machines *machines, pid_t pid)
{
struct machine *machine = machines__findnew(machines, pid);
-
+printf("machines__create_kernel_maps: %p\n", machine);
if (machine == NULL)
return -1;
@@ -1763,7 +1763,7 @@
bid = &__bid;
build_id__init(bid, event->mmap2.build_id, event->mmap2.build_id_size);
}
-
+printf("%s: cpumode %d\n", "machine__process_mmap2_event", sample->cpumode);
if (sample->cpumode == PERF_RECORD_MISC_GUEST_KERNEL ||
sample->cpumode == PERF_RECORD_MISC_KERNEL) {
struct extra_kernel_map xm = {
@@ -1821,6 +1821,7 @@
if (dump_trace)
perf_event__fprintf_mmap(event, stdout);
+printf("%s: cpumode %d\n", "machine__process_mmap_event", sample->cpumode);
if (sample->cpumode == PERF_RECORD_MISC_GUEST_KERNEL ||
sample->cpumode == PERF_RECORD_MISC_KERNEL) {
struct extra_kernel_map xm = {
diff -ur -x '*.o' -x '*.cmd' perf-5.13.0-rc2.orig/tools/perf/util/session.c perf-5.13.0-rc2/tools/perf/util/session.c
--- perf-5.13.0-rc2.orig/tools/perf/util/session.c 2021-05-19 15:04:47.000000000 +0100
+++ perf-5.13.0-rc2/tools/perf/util/session.c 2021-05-20 12:17:31.251730384 +0100
@@ -243,14 +243,14 @@
session->machines.host.single_address_space =
perf_env__single_address_space(session->machines.host.env);
- if (!data || perf_data__is_write(data)) {
+// if (!data || perf_data__is_write(data)) {
/*
* In O_RDONLY mode this will be performed when reading the
* kernel MMAP event, in perf_event__process_mmap().
*/
if (perf_session__create_kernel_maps(session) < 0)
pr_warning("Cannot read kernel map\n");
- }
+// }
/*
* In pipe-mode, evlist is empty until PERF_RECORD_HEADER_ATTR is
--
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