Re: [PATCH] powerpc/interrupt: Don't read MSR from interrupt_exit_kernel_prepare()
From: "Nicholas Piggin" <npiggin@gmail.com>
Date: 2023-06-06 08:57:39
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On Mon Jun 5, 2023 at 6:55 PM AEST, Christophe Leroy wrote:
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.
I did have a patch that warns if you do a local_irq_disable() when irqs are disabled which is why I did this, but it is kind of silly. You could do 'if (!irqs_disabled()) local_irq_disable()' Unfortunately that adds another branch but if it is not taken frequently then maybe avoiding the mtMSR/EID would make it a win? If you don't change that I might end up doing it if I can get that warning patch merged (needs a few core kernel changes). I wonder how much that would help local_irq_save too, if interrupts are already disabled then avoid the mt? Maybe those things are not very costly on smaller in-order cores. Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
quoted hunk ↗ jump to hunk
Fixes: 13799748b957 ("powerpc/64: use interrupt restart table to speed up return from interrupt") Signed-off-by: Christophe Leroy <redacted> --- 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 e34c72285b4e..f3fc5fe919d9 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. */-- 2.40.1