[MODERATED] Re: [RFC PATCH 4/6] kvm: handle host mode irqs 4

[Nicolai Stange <nstange@suse.de>]

speck for Peter Zijlstra  <speck@linutronix.de> writes:

> On Wed, Jul 25, 2018 at 01:45:32PM +0200, speck for Nicolai Stange wrote:
>> So if nobody objects, I'll go with what Paolo proposed at [1]: replace
>> the 'kernel_mode_irq_gen' counter with a latch flag and set that for any
>> irq (but NMIs for a start).
>
> Well, the reason I suggested the other counters, is to avoid adding yet
> another dirty cacheline to IRQs. Since we're already touching those
> fields for most interrupts anyway.. might as well extend them to cover
> what we need instead of adding more dirty lines.
>

Ah I see, that makes sense of course.

I think there are two possible ways forward:

1.) Squeeze that new "kvm_cpu_l1tf_flush_l1d" latch flag into
    irq_cpustat_t. irq_cpustat_t is arch-specific and there
    is some room in its first cacheline, for example inserting
    up to four bytes right after ->irq_tlb_count wouldn't move
    anything critical out.

2.) Make the non-do_IRQ() irqs, i.e. smp_apic_timer_interrupt() & Co,
    call kstat_incr_irqs_this_cpu() and adapt arch_irq_stat_cpu()
    accordingly. Let vmx use struct kernel_stat ->irqs_sum as a
    irq generation counter as before.

Pros 1.)
- Easy to implement.
- Doesn't rely on irq_domain drivers.

Cons 1.)
- Extra store from do_IRQ() irqs in comparison to 2.)
- That "kvm_cpu_l1tf_flush_l1d" flag doesn't really qualify
  as an "IRQ stat" and it might be surprising to have it there.

Pros 2.)
- Might be cleaner, i.e. no random KVM specific flag in irq_cpustat_t.
- Can perhaps be reused in non-KVM contexts, should there ever be a need
  for it.

Cons 2.)
- Slightly more implementation work, i.e. fiddling with
  arch_irq_stat_cpu().
- Relies on irq_domain drivers in that whatever they've installed
  at struct irq_desc ->handle_irq() does a kstat_incr_irqs_this_cpu().

I'm undecided. Thoughts?

Thanks,

Nicolai

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