Derek Atkins wrote: > Dave Anderson <anderson at redhat.com> writes: > > >>>Hi Eric and others, >>> >>>I think we might be running into the issues because i386, FC7 relocatable >>>kernel has been compiled for 16MB physical address but effectively it >>>runs at 4MB physical address. So kernel does not run at compiled address >>>and any kind of debugging tools reading symbol address from System.map >>>or rom vmlinux will fail as run time symbol addresses are different. >>> >>>/proc/kallsyms should help though. This is one problem with shift in run >>>time virtual address while relocating the kernel. We should be running kernel >>>at compiled address to be able to debug it. Or enable any tools to parse >>>/proc/kallsyms to read the shift in symbol addresses and adjust accordingly. >>> >>>Thanks >>>Vivek >> >>Right, crash was updated in version 4.0-4.5 to allow the use >>of /proc/kallsyms as an alternative to the System.map file, >>as well as adding a new --reloc command line argument. After >>bringing up the vmlinux file in gdb (with the "wrong" addresses), >>all of the minimal_symbol data structures in the gdb module are >>back-patched with the /proc/kallsyms values: >> >> http://people.redhat.com/anderson/crash.changelog.html#4_0_4_5 >> >>It seems the benefit of configuring the kernel that way is debatable, >>and I will do all I can to convince the RHEL-6 and beyond kernel >>maintainers from doing it that way in the future. But Fedora goes >>its own way. Seems totally lame to issue a bogus System.map file >>though... > > > Well, this is kgdb, so "/proc/kallsyms" is on the target machine, > not the host machine. So, 'gdb' cannot read /proc/kallsyms, because, > well, it's not local. Well, you could just grab a copy of it, although I don't know what gdb alone would be able to do with it... > > However, I AM building my own kernel -- so I can certainly reconfigure > it as necessary. What do I need to do to reconfigure my kernel to > run at the same place it was built for? I.e., what's changing the > runtime from 16M to 4M and how do I make it consistent? > > Thanks! Configure the kernel with CONFIG_PHYSICAL_START less than or equal to CONFIG_PHYSICAL_ALIGN. Upon rebuilding my FC7 kernel with CONFIG_PHYSICAL_START changed from 16MB to 1MB, with CONFIG_PHYSICAL_ALIGN left at 4MB, i.e.: CONFIG_PHYSICAL_START=0x100000 CONFIG_PHYSICAL_ALIGN=0x400000 The kernel gets compiled for, and runs at, a 4MB physical address: $ nm -Bn vmlinux | grep "^c04" c0400000 T _text c0400000 T startup_32 c0401000 T startup_32_smp c0401080 t checkCPUtype c0401101 t is486 ... Setting both of them to 0x400000 also works. Dave
