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Subject: Re: [PATCH v3 28/32] powerpc/64s: interrupt implement exit logic in C
To: Nicholas Piggin <npiggin@gmail.com>, linuxppc-dev@lists.ozlabs.org
References: <20200225173541.1549955-1-npiggin@gmail.com>
 <20200225173541.1549955-29-npiggin@gmail.com>
From: Christophe Leroy <christophe.leroy@csgroup.eu>
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Date: Wed, 3 Feb 2021 17:25:57 +0100
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Le 25/02/2020 à 18:35, Nicholas Piggin a écrit :
> Implement the bulk of interrupt return logic in C. The asm return code
> must handle a few cases: restoring full GPRs, and emulating stack store.
> 


> +notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs, unsigned long msr)
> +{
> +	unsigned long *ti_flagsp = &current_thread_info()->flags;
> +	unsigned long flags;
> +
> +	if (IS_ENABLED(CONFIG_PPC_BOOK3S) && unlikely(!(regs->msr & MSR_RI)))
> +		unrecoverable_exception(regs);
> +	BUG_ON(regs->msr & MSR_PR);
> +	BUG_ON(!FULL_REGS(regs));
> +
> +	local_irq_save(flags);
> +
> +	if (regs->softe == IRQS_ENABLED) {
> +		/* Returning to a kernel context with local irqs enabled. */
> +		WARN_ON_ONCE(!(regs->msr & MSR_EE));
> +again:
> +		if (IS_ENABLED(CONFIG_PREEMPT)) {
> +			/* Return to preemptible kernel context */
> +			if (unlikely(*ti_flagsp & _TIF_NEED_RESCHED)) {
> +				if (preempt_count() == 0)
> +					preempt_schedule_irq();
> +			}
> +		}
> +
> +		trace_hardirqs_on();
> +		__hard_EE_RI_disable();
> +		if (unlikely(lazy_irq_pending())) {
> +			__hard_RI_enable();
> +			irq_soft_mask_set(IRQS_ALL_DISABLED);
> +			trace_hardirqs_off();
> +			local_paca->irq_happened |= PACA_IRQ_HARD_DIS;
> +			/*
> +			 * Can't local_irq_enable in case we are in interrupt
> +			 * context. Must replay directly.
> +			 */
> +			replay_soft_interrupts();
> +			irq_soft_mask_set(flags);
> +			/* Took an interrupt, may have more exit work to do. */
> +			goto again;
> +		}
> +		local_paca->irq_happened = 0;
> +		irq_soft_mask_set(IRQS_ENABLED);
> +	} else {
> +		/* Returning to a kernel context with local irqs disabled. */
> +		trace_hardirqs_on();
> +		__hard_EE_RI_disable();
> +		if (regs->msr & MSR_EE)
> +			local_paca->irq_happened &= ~PACA_IRQ_HARD_DIS;
> +	}
> +
> +
> +#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
> +	local_paca->tm_scratch = regs->msr;
> +#endif
> +
> +	/*
> +	 * We don't need to restore AMR on the way back to userspace for KUAP.
> +	 * The value of AMR only matters while we're in the kernel.
> +	 */
> +	kuap_restore_amr(regs);

Is that correct to restore KUAP state here ? Shouldn't we have it at lower level in assembly ?

Isn't there a risk that someone manages to call interrupt_exit_kernel_prepare() or the end of it in 
a way or another, and get the previous KUAP state restored by this way ?

Also, it looks a bit strange to have kuap_save_amr_and_lock() done at lowest level in assembly, and 
kuap_restore_amr() done in upper level. That looks unbalanced.

Christophe


> +
> +	if (unlikely(*ti_flagsp & _TIF_EMULATE_STACK_STORE)) {
> +		clear_bits(_TIF_EMULATE_STACK_STORE, ti_flagsp);
> +		return 1;
> +	}
> +	return 0;
> +}
> +#endif
> diff --git a/arch/powerpc/kernel/vector.S b/arch/powerpc/kernel/vector.S
> index 25c14a0981bf..d20c5e79e03c 100644
> --- a/arch/powerpc/kernel/vector.S
> +++ b/arch/powerpc/kernel/vector.S
> @@ -134,7 +134,7 @@ _GLOBAL(load_up_vsx)
>   	/* enable use of VSX after return */
>   	oris	r12,r12,MSR_VSX@h
>   	std	r12,_MSR(r1)
> -	b	fast_exception_return
> +	b	fast_interrupt_return
>   
>   #endif /* CONFIG_VSX */
>   
>