[PATCH v3 1/9] powerpc/kuap: Avoid unnecessary reads of MD_AP

From: Christophe Leroy
Date: Tue Jul 11 2023 - 11:59:54 EST


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@xxxxxxxxxx>
---
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,
--
2.41.0