Re: [PATCH] powerpc: Remove MSR_PR check in interrupt_exit_{user/kernel}_prepare()

From: Christophe Leroy
Date: Wed Aug 04 2021 - 04:37:16 EST




Le 04/08/2021 à 10:08, Nicholas Piggin a écrit :
Excerpts from Christophe Leroy's message of August 4, 2021 3:27 pm:
In those hot functions that are called at every interrupt, any saved
cycle is worth it.

interrupt_exit_user_prepare() and interrupt_exit_kernel_prepare() are
called from three places:
- From entry_32.S
- From interrupt_64.S
- From interrupt_exit_user_restart() and interrupt_exit_kernel_restart()

In entry_32.S, there are inambiguously called based on MSR_PR:

interrupt_return:
lwz r4,_MSR(r1)
addi r3,r1,STACK_FRAME_OVERHEAD
andi. r0,r4,MSR_PR
beq .Lkernel_interrupt_return
bl interrupt_exit_user_prepare
...
.Lkernel_interrupt_return:
bl interrupt_exit_kernel_prepare

In interrupt_64.S, that's similar:

interrupt_return_\srr\():
ld r4,_MSR(r1)
andi. r0,r4,MSR_PR
beq interrupt_return_\srr\()_kernel
interrupt_return_\srr\()_user: /* make backtraces match the _kernel variant */
addi r3,r1,STACK_FRAME_OVERHEAD
bl interrupt_exit_user_prepare
...
interrupt_return_\srr\()_kernel:
addi r3,r1,STACK_FRAME_OVERHEAD
bl interrupt_exit_kernel_prepare

In interrupt_exit_user_restart() and interrupt_exit_kernel_restart(),
MSR_PR is verified respectively by BUG_ON(!user_mode(regs)) and
BUG_ON(user_mode(regs)) prior to calling interrupt_exit_user_prepare()
and interrupt_exit_kernel_prepare().

The verification in interrupt_exit_user_prepare() and
interrupt_exit_kernel_prepare() are therefore useless and can be removed.

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>

Probably okay to do now things are ironing out.

Unless we want to make a new define for interrupt handler debug and put
a bunch of these asserts under it. There's quite a lot more here, and
in asm/interrupt.h, etc.

But that one is so trivial that I'm not sure there is any point in keeping it even as a kind of additional DEBUG level, unless you want those BUG_ONs because you don't trust the compiler.

Christophe


Thanks,
Nick

---
arch/powerpc/kernel/interrupt.c | 2 --
1 file changed, 2 deletions(-)

diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 21bbd615ca41..f26caf911ab5 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -465,7 +465,6 @@ notrace unsigned long interrupt_exit_user_prepare(struct pt_regs *regs)
if (!IS_ENABLED(CONFIG_BOOKE) && !IS_ENABLED(CONFIG_40x))
BUG_ON(!(regs->msr & MSR_RI));
- BUG_ON(!(regs->msr & MSR_PR));
BUG_ON(arch_irq_disabled_regs(regs));
CT_WARN_ON(ct_state() == CONTEXT_USER);
@@ -499,7 +498,6 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
if (!IS_ENABLED(CONFIG_BOOKE) && !IS_ENABLED(CONFIG_40x) &&
unlikely(!(regs->msr & MSR_RI)))
unrecoverable_exception(regs);
- BUG_ON(regs->msr & MSR_PR);
/*
* CT_WARN_ON comes here via program_check_exception,
* so avoid recursion.
--
2.25.0