On Thu, May 08, 2025 at 01:23:04PM -0700, H. Peter Anvin wrote:

On May 8, 2025 5:15:44 AM PDT, Peter Zijlstra [off-list ref] wrote:

quoted

quoted

Looks good, except when fred_event_data() returns 0. I don't expect it
to happen in practice. But, maybe with new hardware and eventually
different hypervisors being involved, it is a possibility.

We can either call it a bug that an NMI happened without source
information. Or be extra nice and do this:

if (cpu_feature_enabled(X86_FEATURE_NMI_SOURCE) && type == NMI_LOCAL) {
	source = fred_event_data(regs);
	if (!source || (source & BIT(0)))
		source = ~0UL;
}

Perhaps also WARN about the !source case?

A 0 should be interpreted such that NMI source is not available, e.g.
due to a broken hypervisor or similar.

I'm reading that as an agreement for WARN-ing on 0. We should definitely
WARN on broken hypervisors and similar.