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

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Date:	Wed, 25 Mar 2015 21:14:25 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 0/3] x86: do not save callee-preserved registers around lockdep_sys_exit_thunk

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@...ux-foundation.org>
CC: Steven Rostedt <rostedt@...dmis.org>
CC: Ingo Molnar <mingo@...nel.org>
CC: Borislav Petkov <bp@...en8.de>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: Andy Lutomirski <luto@...capital.net>
CC: Oleg Nesterov <oleg@...hat.com>
CC: Frederic Weisbecker <fweisbec@...il.com>
CC: Alexei Starovoitov <ast@...mgrid.com>
CC: Will Drewry <wad@...omium.org>
CC: Kees Cook <keescook@...omium.org>
CC: x86@...nel.org
CC: linux-kernel@...r.kernel.org

-- 
1.8.1.4

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