linux-kernel - [PATCH 1/4] powerpc/kuap: Avoid unnecessary reads of MD

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Message-Id: <1a3c69e38349b687c6c65240d7c09a7817a797d8.1685963081.git.christophe.leroy@csgroup.eu>
Date:   Mon,  5 Jun 2023 13:04:50 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH 1/4] powerpc/kuap: Avoid unnecessary reads of MD_AP

A disassembly of interrupt_exit_kernel_prepare() shows a useless read
of MD_AP register. This is shown by r9 being re-used immediately without
doing anything with the value read.

  c000e0e0:       60 00 00 00     nop
  c000e0e4: ===>  7d 3a c2 a6     mfmd_ap r9	<====
  c000e0e8:       7d 20 00 a6     mfmsr   r9
  c000e0ec:       7c 51 13 a6     mtspr   81,r2
  c000e0f0:       81 3f 00 84     lwz     r9,132(r31)
  c000e0f4:       71 29 80 00     andi.   r9,r9,32768

kuap_get_and_assert_locked() is paired with kuap_kernel_restore()
and are only used in interrupt_exit_kernel_prepare(). The value
returned by kuap_get_and_assert_locked() is only used by
kuap_kernel_restore().

On 8xx, kuap_kernel_restore() doesn't use the value read by
kuap_get_and_assert_locked() so modify kuap_get_and_assert_locked()
to not perform the read of MD_AP and return 0 instead.

The same applies on BOOKE.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/include/asm/nohash/32/kup-8xx.h | 8 ++------
 arch/powerpc/include/asm/nohash/kup-booke.h  | 6 ++----
 2 files changed, 4 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/include/asm/nohash/32/kup-8xx.h b/arch/powerpc/include/asm/nohash/32/kup-8xx.h
index c44d97751723..8579210f2a6a 100644
--- a/arch/powerpc/include/asm/nohash/32/kup-8xx.h
+++ b/arch/powerpc/include/asm/nohash/32/kup-8xx.h
@@ -41,14 +41,10 @@ static inline void __kuap_kernel_restore(struct pt_regs *regs, unsigned long kua
 
 static inline unsigned long __kuap_get_and_assert_locked(void)
 {
-	unsigned long kuap;
-
-	kuap = mfspr(SPRN_MD_AP);
-
 	if (IS_ENABLED(CONFIG_PPC_KUAP_DEBUG))
-		WARN_ON_ONCE(kuap >> 16 != MD_APG_KUAP >> 16);
+		WARN_ON_ONCE(mfspr(SPRN_MD_AP) >> 16 != MD_APG_KUAP >> 16);
 
-	return kuap;
+	return 0;
 }
 
 static inline void __allow_user_access(void __user *to, const void __user *from,
diff --git a/arch/powerpc/include/asm/nohash/kup-booke.h b/arch/powerpc/include/asm/nohash/kup-booke.h
index 49bb41ed0816..823c5a3a96d8 100644
--- a/arch/powerpc/include/asm/nohash/kup-booke.h
+++ b/arch/powerpc/include/asm/nohash/kup-booke.h
@@ -58,12 +58,10 @@ static inline void __kuap_kernel_restore(struct pt_regs *regs, unsigned long kua
 
 static inline unsigned long __kuap_get_and_assert_locked(void)
 {
-	unsigned long kuap = mfspr(SPRN_PID);
-
 	if (IS_ENABLED(CONFIG_PPC_KUAP_DEBUG))
-		WARN_ON_ONCE(kuap);
+		WARN_ON_ONCE(mfspr(SPRN_PID));
 
-	return kuap;
+	return 0;
 }
 
 static inline void __allow_user_access(void __user *to, const void __user *from,
-- 
2.40.1