Meaning of the [k] symbol in perf report output

Question

What is the meaning of the [k] symbol I can see in the following perf report output:

+   63.93%     0.10%  sample    libc.so.6              [.] __GI___fstatat64
+   63.82%    63.82%  sample    [unknown]              [k] 0xffffffffa90001a2     <---- here
+    5.32%     0.00%  sample    [unknown]              [.] 0000000000000000

I was unable to find any reference to this on the perf-report man page here - https://man7.org/linux/man-pages/man1/perf-report.1.html

Answer 1

As helpfully provided by @Barmar, it's documented here:

https://perf.wiki.kernel.org/index.php/Tutorial#Sample_analysis_with_perf_report:

[.] : user level
[k]: kernel level
[g]: guest kernel level (virtualization)
[u]: guest os user space
[H]: hypervisor