On Thu, Jan 21, 2021 at 09:52:01AM +0100, Andrea Righi wrote: > On Thu, Jan 21, 2021 at 08:49:28AM +0100, Ingo Molnar wrote: > > > > * tip-bot2 for Andrea Righi <tip-b...@linutronix.de> wrote: > > > > > The following commit has been merged into the x86/entry branch of tip: > > > > > > Commit-ID: e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89 > > > Gitweb: > > > https://git.kernel.org/tip/e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89 > > > Author: Andrea Righi <andrea.ri...@canonical.com> > > > AuthorDate: Thu, 14 Jan 2021 12:48:35 +01:00 > > > Committer: Ingo Molnar <mi...@kernel.org> > > > CommitterDate: Thu, 21 Jan 2021 08:11:52 +01:00 > > > > > > x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y > > > > > > With CONFIG_PREEMPTION disabled, arch/x86/entry/thunk_64.o is just an > > > empty object file. > > > > > > With the newer binutils (tested with 2.35.90.20210113-1ubuntu1) the GNU > > > assembler doesn't generate a symbol table for empty object files and > > > objtool fails with the following error when a valid symbol table cannot > > > be found: > > > > > > arch/x86/entry/thunk_64.o: warning: objtool: missing symbol table > > > > > > To prevent this from happening, build thunk_$(BITS).o only if > > > CONFIG_PREEMPTION is enabled. > > > > > > BugLink: https://bugs.launchpad.net/bugs/1911359 > > > > > > Fixes: 320100a5ffe5 ("x86/entry: Remove the TRACE_IRQS cruft") > > > Signed-off-by: Andrea Righi <andrea.ri...@canonical.com> > > > Signed-off-by: Ingo Molnar <mi...@kernel.org> > > > Cc: Borislav Petkov <b...@alien8.de> > > > Link: https://lore.kernel.org/r/YAAvk0UQelq0Ae7+@xps-13-7390 > > > > Hm, this fails to build on UML defconfig: > > > > > > /home/mingo/gcc/cross/lib/gcc/x86_64-linux/9.3.1/../../../../x86_64-linux/bin/ld: > > arch/x86/um/../entry/thunk_64.o: in function `preempt_schedule_thunk': > > /home/mingo/tip.cross/arch/x86/um/../entry/thunk_64.S:34: undefined > > reference to `preempt_schedule' > > > > /home/mingo/gcc/cross/lib/gcc/x86_64-linux/9.3.1/../../../../x86_64-linux/bin/ld: > > arch/x86/um/../entry/thunk_64.o: in function > > `preempt_schedule_notrace_thunk': > > /home/mingo/tip.cross/arch/x86/um/../entry/thunk_64.S:35: undefined > > reference to `preempt_schedule_notrace' > > > > Thanks, > > > > Ingo > > I've been able to reproduce it, I'm looking at this right now. Thanks!
I see, basically UML selects ARCH_NO_PREEMPT, but in arch/x86/um/Makefile it explicitly includes thunk_$(BITS).o regardless. Considering that thunk_$(BITS) only contains preemption code now, we can probably drop it from the Makefile, or, to be more consistent with the x86 change, we could include it only if CONFIG_PREEMPTION is enabled (even if it would never be, because UML has ARCH_NO_PREEMPT). If it's unlikely that preemption will be enabled in UML one day I'd probably go with the former, otherwise I'd go with the latter, because it looks more consistent. Opinions? Thanks, -Andrea
