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Date:   Tue, 09 Feb 2021 12:36:20 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in
 another pointer
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Michael Ellerman <mpe@ellerman.id.au>, msuchanek@suse.de,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <cover.1612796617.git.christophe.leroy@csgroup.eu>
        <24804747098369ebcdac38970b8f7a1260bdd248.1612796617.git.christophe.leroy@csgroup.eu>
In-Reply-To: <24804747098369ebcdac38970b8f7a1260bdd248.1612796617.git.christophe.leroy@csgroup.eu>
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Excerpts from Christophe Leroy's message of February 9, 2021 1:10 am:
> By saving the pointer pointing to thread_info.flags, gcc copies r2
> in a non-volatile register.
>=20
> We know 'current' doesn't change, so avoid that intermediaite pointer.
>=20
> Reduces null_syscall benchmark by 2 cycles (322 =3D> 320 cycles)
>=20
> On PPC64, gcc seems to know that 'current' is not changing, and it keeps
> it in a non volatile register to avoid multiple read of 'current' in paca=
.
>=20
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

What if you did this?

---
 arch/powerpc/include/asm/current.h | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/current.h b/arch/powerpc/include/asm/=
current.h
index bbfb94800415..59ab327972a5 100644
--- a/arch/powerpc/include/asm/current.h
+++ b/arch/powerpc/include/asm/current.h
@@ -23,16 +23,19 @@ static inline struct task_struct *get_current(void)
=20
 	return task;
 }
-#define current	get_current()
=20
 #else
=20
-/*
- * We keep `current' in r2 for speed.
- */
-register struct task_struct *current asm ("r2");
+static inline struct task_struct *get_current(void)
+{
+	register struct task_struct *task asm ("r2");
+
+	return task;
+}
=20
 #endif
=20
+#define current	get_current()
+
 #endif /* __KERNEL__ */
 #endif /* _ASM_POWERPC_CURRENT_H */
--=20