[PATCH 0/3] x86: do not save callee-preserved registers around lockdep_sys_exit_thunk

[Denys Vlasenko <dvlasenk@redhat.com>]

Recent change to struct pt_regs handling in entry.S, among other things,
changed how callee-preserved registers are saved around call
to lockdep_sys_exit_thunk:

 #    define LOCKDEP_SYS_EXIT_IRQ \
        TRACE_IRQS_ON; \
        sti; \
        SAVE_EXTRA_REGS; \    <======= HERE
        LOCKDEP_SYS_EXIT; \
        RESTORE_EXTRA_REGS; \ <======= HERE

The change is in fact a bit fragile:
now we assume that LOCKDEP_SYS_EXIT_IRQ macro is used only
when there is a struct pt_regs on the stack.

So far this assumption is true, but it is probably a coincidence.

We can revert back to pushing registers, but this callee-preserved registers
saving appears bogus. It is not necessary: they _are_ callee-preserved,
and lockdep_sys_exit() does not touch them on the stack either.

First two patches clean up #define maze so that we can see
that LOCKDEP_SYS_EXIT macro call above is merely "call lockdep_sys_exit_thunk".

Last patch removes SAVE/RESTORE.

I tested this patch series and everything seems to run fine
on a lockdep-enabled kernel with these lines removed.

Denys Vlasenko (3):
  x86: move ARCH_LOCKDEP_SYS_EXIT[IRQ] defines closer to their users
  x86: fold ARCH_LOCKDEP_SYS_EXIT defines into their users
  x86: do not save callee-preserved registers around lockdep_sys_exit_thunk

 arch/x86/include/asm/irqflags.h | 45 +++++++++++++++++------------------------
 1 file changed, 19 insertions(+), 26 deletions(-)

CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org

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1.8.1.4