+kbuild maintainers On Tue, May 18, 2021 at 09:50:04PM +0200, Peter Zijlstra wrote: > On Wed, May 12, 2021 at 01:19:47PM -0000, tip-bot2 for Peter Zijlstra wrote: > > The following commit has been merged into the objtool/core branch of tip: > > > > Commit-ID: ab3257042c26d0cd44793c741e2f89bf38b21fe8 > > Gitweb: https://git.kernel.org/tip/ab3257042c26d0cd44793c741e2f89bf38b21fe8 > > Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > AuthorDate: Thu, 06 May 2021 21:34:05 +02:00 > > Committer: Ingo Molnar <mingo@xxxxxxxxxx> > > CommitterDate: Wed, 12 May 2021 14:54:56 +02:00 > > > > jump_label, x86: Allow short NOPs > > > > Now that objtool is able to rewrite jump_label instructions, have the > > compiler emit a JMP, such that it can decide on the optimal encoding, > > and set jump_entry::key bit1 to indicate that objtool should rewrite > > the instruction to a matching NOP. > > > > For x86_64-allyesconfig this gives: > > > > jl\ NOP JMP > > short: 22997 124 > > long: 30874 90 > > > > IOW, we save (22997+124) * 3 bytes of kernel text in hotpaths. > > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> > > Link: https://lore.kernel.org/r/20210506194158.216763632@xxxxxxxxxxxxx > > So Willy is having some trouble with this commit; for some reason his > kernel is no longer booting in his qemu thing, but I can't reproduce. > > I've hacked up the below vmlinux.o validation, willy can you run this on > your vmlinux.o, something like: > > build/tools/objtool/objtool check -abdJsuld build/vmlinux.o > > Where I'm assuming you build with O=build/. When I run it on my build > (with your .config) I get absolutely nothing :/ > > Alternatively, can you get me your vmlinux.o + bzImage ? > > Also helpful might be trying to attach gdb to the qemu gdbstub and > looking where the boot fails. OK, willy followed up on IRC, and it turns out there's a kbuild dependency missing; then objtool changes we don't rebuild: arch/x86/entry/vdso/vma.o even though we should, this led to an unpatched 2 byte jump-label and things went sideways. I'm not sure I understand the whole build machinery well enough to know where to begin chasing this. Now, this file is mighty magical, due to: arch/x86/entry/vdso/Makefile:OBJECT_FILES_NON_STANDARD := y arch/x86/entry/vdso/Makefile:OBJECT_FILES_NON_STANDARD_vma.o := n Maybe that's related.
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