On 12/14/22 12:00, Alex Bennée wrote: > > Alex Bennée <alex.ben...@linaro.org> writes: > >> wanghw364 <wanghw...@163.com> writes: >> >>> Thanks. I have several questions as below, please help, thanks. >>> >>> 1.What do you mean by "only have debug symbols available for linux-user >>> so"? What does the linux-user so >>> refer to? >>> qemu_plugin_insn_symbol() can only see symbols from linux-user so? >> >> The linux-user ELF loader will read the debug symbols (if they exist) >> and populate the syminfos structures that lookup_symbol uses. It's >> partially obscured by the ELF loaders heavy use of macros but see: >> >> static void glue(load_symbols, SZ)(struct elfhdr *ehdr, int fd, int >> must_swab, >> int clear_lsb, symbol_fn_t sym_cb) >> >> in elf_ops.h >> >>> 2.The purpose of teaching the linux kernel loader to understand and >>> relocate symbols from an ELF kernel >>> image, >>> or extract then and feed them directly to the plugin, is to solve >>> the issue that qemu_plugin_insn_symbol() >>> can't see kernel symbol? >> >> Yes. This is slightly complicated by the fact that the kernel loaders don't >> expect to load pure ELF files but something that is wrapped up as a >> Linux loader. For example: >> >> ➜ file vmlinux >> vmlinux: ELF 64-bit LSB pie executable, ARM aarch64, version 1 >> (SYSV), statically linked, >> BuildID[sha1]=21166458a10404e6157abf0da4a0921144c72675, with >> debug_info, not stripped >> 🕙10:07:42 alex@zen:linux.git/builds/arm64.initramfs with >> arm64/aarch64-linux-gnu- on linux-6.0.y [$!?] >> ➜ file arch/arm64/boot/Image >> arch/arm64/boot/Image: Linux kernel ARM64 boot executable Image, >> little-endian, 4K pages >> >> The second file is what is actually passed to -kernel in a typical boot. >> >> The logic in arm_setup_direct_kernel_boot() implies you can load ELFs >> directly and boot them but for some reason the Linux kernel doesn't work >> if you try this way. > > Replying to myself - this is because the vmlinux image is based of > kernel virtual address. So the import thing the loader does is create > the initial vaddr mappings and relocate the kernel to that location > before running it. See the function primary_entry in head.S in the > kernel. > > So perhaps for system emulation it would be useful to have a -symbols > option to load symbols from another file. >
Hi Alex, it doesn't need to be a tcg plugin-only feature right, it's possible to use qemu to debug the guest also when using KVM.. Ciao, Claudio
