On Sat, Sep 25, 2021 at 11:39:55PM +0800, Pingfan Liu wrote:

On Fri, Sep 24, 2021 at 06:53:06PM +0100, Mark Rutland wrote:

quoted

On Fri, Sep 24, 2021 at 09:28:33PM +0800, Pingfan Liu wrote:

quoted

In enter_el1_irq_or_nmi(), it can be the case which NMI interrupts an
irq, which makes the condition !interrupts_enabled(regs) fail to detect
the NMI. This will cause a mistaken account for irq.

Sorry about the confusing word "account", it should be "lockdep/rcu/.."

quoted

Can you please explain this in more detail? It's not clear which
specific case you mean when you say "NMI interrupts an irq", as that
could mean a number of distinct scenarios.

AFAICT, if we're in an IRQ handler (with NMIs unmasked), and an NMI
causes a new exception we'll do the right thing. So either I'm missing a
subtlety or you're describing a different scenario..

Note that the entry code is only trying to distinguish between:

a) This exception is *definitely* an NMI (because regular interrupts
   were masked).

b) This exception is *either* and IRQ or an NMI (and this *cannot* be
   distinguished until we acknowledge the interrupt), so we treat it as
   an IRQ for now.

b) is the aim.

At the entry, enter_el1_irq_or_nmi() -> enter_from_kernel_mode()->rcu_irq_enter()/rcu_irq_enter_check_tick() etc.
While at irqchip level, gic_handle_irq()->gic_handle_nmi()->nmi_enter(),
which does not call rcu_irq_enter_check_tick(). So it is not proper to
"treat it as an IRQ for now"

I'm struggling to understand the problem here. What is "not proper", and
why?

Do you think there's a correctness problem, or that we're doing more
work than necessary? 

If you could give a specific example of a problem, it would really help.

I'm aware that we do more work than strictly necessary when we take a
pNMI from a context with IRQs enabled, but that's how we'd intended this
to work, as it's vastly simpler to manage the state that way. Unless
there's a real problem with that approach I'd prefer to leave it as-is.

Thanks,
Mark.

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