Re: [tip: x86/entry] x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y

[Andrea Righi <andrea.righi@canonical.com>]

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-bot2@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.righi@canonical.com>
> > > AuthorDate:    Thu, 14 Jan 2021 12:48:35 +01:00
> > > Committer:     Ingo Molnar <mingo@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.righi@canonical.com>
> > > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > > Cc: Borislav Petkov <bp@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