Re: [PATCH RFC] x86/cpu: fix intermittent lockup on poweroff

[Thomas Gleixner <tglx@linutronix.de>]

Andi!

On Wed, Apr 26 2023 at 15:02, Andi Kleen wrote:
>> > > This is probably going to pull in a cache line and cause the problem the
>> > > native_wbinvd() is trying to avoid.
>> > 
>> > Is one _more_ cacheline really the problem?
>> 
>> The answer is it depends. If the cacheline ends up modified/dirty, then it
>> can be a problem.
>
> I haven't followed this all in detail, but if any dirty cache line a
> problem you probably would need to be sure that any possible NMI user
> (like perf or watchdogs) is disabled at this point, otherwise you could
> still get NMIs here. 
>
> I don't think perf currently has a mechanism to do that other
> than to offline the CPU.

stop_this_cpu()
  disable_local_APIC()
    apic_soft_disable()
      clear_local_APIC()
        v = apic_read(APIC_LVTPC);
        apic_write(APIC_LVTPC, v | APIC_LVT_MASKED);

So after that point the PMU can't raise NMIs anymore which includes the
default perf based NMI watchdog, no?

External NMIs are a different problem, but they kinda fall into the same
category as:

> Also there are of course machine checks and SMIs that could still happen, 
> but I guess there's nothing you could do about them.

Though external NMIs could be disabled via outb(0x70,...) if paranoid
enough. Albeit if there is an external NMI based watchdog raising the
NMI during this kexec() scenario then the outcome is probably as
undefined as in the MCE case independent of the SEV dirty cacheline
concern.

Thanks,

        tglx