Re: [PATCH v5 5/9] x86/nmi: Add support to handle NMIs with source information

Peter Zijlstra
2025-05-08T20:49:26Z

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 <peterz@infradead.org> wrote:

> >> 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.