Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Jason Gerecke <killertofu@gmail.com>
To:     linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jason Baron <jbaron@akamai.com>
Cc:     Jason Gerecke <jason.gerecke@wacom.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ard Biesheuvel <ardb@kernel.org>
Subject: [PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints
Date:   Thu, 11 Feb 2021 13:48:48 -0800
Message-Id: <20210211214848.536626-1-jason.gerecke@wacom.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Precedence: bulk

When compiling an external kernel module with `-O0` or `-O1`, the following
compile error may be reported:

    ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’
       25 |  asm_volatile_goto("1:"
          |  ^~~~~~~~~~~~~~~~~

It appears that these lower optimization levels prevent GCC from detecting
that the key/branch arguments can be treated as constants and used as
immediate operands. To work around this, explicitly add the `const` label.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
---
Marked RFC since I'm not familiar with this subsystem or the asm blocks that
are impacted. Extra-close inspection would be appreciated.

 arch/x86/include/asm/jump_label.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 06c3cc22a058..7f2006645d84 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -20,7 +20,7 @@
 #include <linux/stringify.h>
 #include <linux/types.h>
 
-static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
+static __always_inline bool arch_static_branch(struct static_key * const key, const bool branch)
 {
 	asm_volatile_goto("1:"
 		".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
@@ -36,7 +36,7 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
 	return true;
 }
 
-static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
+static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)
 {
 	asm_volatile_goto("1:"
 		".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
-- 
2.30.1