[PATCH 6.4 464/800] powerpc/interrupt: Dont read MSR from interrupt_exit_kernel_prepare()

Greg Kroah-Hartman
2023-07-16T20:16:02Z

From: Christophe Leroy <christophe.leroy@csgroup.eu>

[ Upstream commit 0eb089a72fda3f7969e6277804bde75dc1474a14 ]

A disassembly of interrupt_exit_kernel_prepare() shows a useless read
of MSR 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

This is due to the use of local_irq_save(). The flags read by
local_irq_save() are never used, use local_irq_disable() instead.

Fixes: 13799748b957 ("powerpc/64: use interrupt restart table to speed up return from interrupt")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/df36c6205ab64326fb1b991993c82057e92ace2f.1685955214.git.christophe.leroy@csgroup.eu
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/powerpc/kernel/interrupt.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index e34c72285b4e9..f3fc5fe919d96 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -368,7 +368,6 @@ void preempt_schedule_irq(void);

notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
{
- unsigned long flags;
unsigned long ret = 0;
unsigned long kuap;
bool stack_store = read_thread_flags() & _TIF_EMULATE_STACK_STORE;
@@ -392,7 +391,7 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)

kuap = kuap_get_and_assert_locked();

- local_irq_save(flags);
+ local_irq_disable();

if (!arch_irq_disabled_regs(regs)) {
/* Returning to a kernel context with local irqs enabled. */
--
2.39.2