I have one virtual machine running on QEMU/KVM and I'm trying to profile QEMU to investigate performance related issues. I have it compiled on a Linux system with debug symbols -
mkdir build
cd build
../configure --target-list=x86_64-softmmu --enable-debug-info
make -j`nproc`
Running perf on the QEMU process does list most of the symbols, but some are showing up as unknown -
perf record -g --quiet -p $qemu_pid -o perf.out -F 99 -- sleep 10 &
perf report -
+ 74.83% 0.00% worker [kernel.kallsyms] [k] entry_SYSCALL_64_after_hwframe ◆
+ 74.83% 0.00% worker [kernel.kallsyms] [k] do_syscall_64 ▒
- 72.37% 0.00% worker [unknown] [k] 0000000000000000 ▒
- 0 ▒
- 32.87% 0x6a00000002 ▒
+ __libc_pwrite ▒
- 21.18% 0xb00000001 ▒
__libc_pread ▒
entry_SYSCALL_64_after_hwframe ▒
+ do_syscall_64 ▒
+ 13.43% pwritev64 ▒
+ 4.50% fdatasync ▒
+ 44.75% 0.00% worker [kernel.kallsyms] [k] ext4_file_write_iter ▒
+ 44.71% 0.00% worker [kernel.kallsyms] [k] ext4_buffered_write_iter ▒
+ 33.36% 0.00% worker [kernel.kallsyms] [k] vfs_write ▒
+ 32.87% 0.00% worker [unknown] [k] 0x0000006a00000002 ▒
+ 32.87% 0.00% worker libc.so.6 [.] __libc_pwrite ▒
+ 32.74% 0.00% worker [kernel.kallsyms] [k] __x64_sys_pwrite64 ▒
+ 31.48% 0.00% worker [kernel.kallsyms] [k] new_sync_write ▒
+ 31.17% 0.00% worker [kernel.kallsyms] [k] generic_perform_write ▒
+ 21.27% 0.00% worker libc.so.6 [.] __libc_pread ▒
+ 21.22% 0.00% worker [kernel.kallsyms] [k] ext4_file_read_iter ▒
+ 21.22% 0.00% worker [kernel.kallsyms] [k] generic_file_read_iter ▒
+ 21.22% 0.37% worker [kernel.kallsyms] [k] filemap_read ▒
+ 21.18% 0.00% worker [unknown] [k] 0x0000000b00000001
...etc
Is there a way to fix the unknown symbols to generate a complete perf report? Or any way to know which library these symbols may belong to, so that it can be compiled with debug symbols?
QEMU version -
user@host:~$ /usr/bin/qemu-system-x86_64 --version
QEMU emulator version 6.2.0 (Debian 1:6.2+dfsg-2ubuntu6.16)
Copyright (c) 2003-2021 Fabrice Bellard and the QEMU Project developers
QEMU documentation does not mention the flags to enable for building a non-stripped binary.
Running ./configure -h will list all the available options and after some investigation, I was able to build the binary with debug symbols.
./configure --enable-debug-tcg --enable-debug-info --enable-debug --disable-strip --target-list=x86_64-softmmu --enable-kvm --extra-cflags="-g3" --extra-ldflags="-g3"