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Message-Id: <d6de17e1bd51292ccacacd4c26b7753f61ed9e40.1687430631.git.christophe.leroy@csgroup.eu>
Date: Thu, 22 Jun 2023 12:54:23 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Michael Ellerman <mpe@...erman.id.au>,
Nicholas Piggin <npiggin@...il.com>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Sathvika Vasireddy <sv@...ux.ibm.com>,
Naveen N Rao <naveen@...nel.org>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v2 01/14] 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
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