Re: [PATCH 3/6] perf/x86/intel/pt: Stop tracing at syscore shutdown

[Andi Kleen <ak@linux.intel.com>]

> +
> +/*
> + * Tracing must not continue into a kexec'ed kernel, which may reuse the
> + * trace buffer memory, so stop tracing on all CPUs and prevent it from
> + * being started again. Events are left in place, but record nothing more.
> + */
> +static void pt_syscore_shutdown(void *data)
> +{
> +	WRITE_ONCE(pt_shutdown, true);
> +	on_each_cpu(pt_shutdown_cpu, NULL, 1);

I don't think adding more and more broadcast NMIs to the panic path 
is a good idea. It's always a very big hammer and a risky operation
that risks breaking the actual dump. panic reallhy needs to be
simple and reliable, and this is the total opposite.

KVM already does one at least (which I never liked either). At a
minimum please find a way to consolidate those into a single
one (as in have a notifier chain that is called on the target CPU).

But better would be to not do this and find some other way.

How about just passing the PT region as reserved and maybe free
it later after the target kernel had a chance to reinitialize all CPUs?


-Andi