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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 07, 2025 at 02:48:34PM -0700, Sohil Mehta wrote:
> On 5/7/2025 2:14 AM, Peter Zijlstra wrote:
> > On Tue, May 06, 2025 at 06:21:41PM -0700, Sohil Mehta wrote:
> >>
> >> diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
> >> index a1d672dcb6f0..183e3e717326 100644
> >> --- a/arch/x86/kernel/nmi.c
> >> +++ b/arch/x86/kernel/nmi.c
> > 
> >>  static int nmi_handle(unsigned int type, struct pt_regs *regs)
> >>  {
> >>  	struct nmi_desc *desc = nmi_to_desc(type);
> >> +	unsigned long source_bitmap = 0;
> > 
> > 	unsigned long source = ~0UL;
> > 
> 
> Thanks! This makes the logic even simpler by getting rid of
> match_nmi_source(). A minor change described further down.
> 
> Also, do you prefer "source" over "source_bitmap"? I had it as such to
> avoid confusion between source_vector and source_bitmap.

Yeah, I was lazy typing. Perhaps just call it bitmap then?

> >>  	nmi_handler_t ehandler;
> >>  	struct nmiaction *a;
> >>  	int handled=0;
> >> @@ -148,16 +164,40 @@ static int nmi_handle(unsigned int type, struct pt_regs *regs)
> >>  
> >>  	rcu_read_lock();
> >>  
> >> +	/*
> >> +	 * Activate NMI source-based filtering only for Local NMIs.
> >> +	 *
> >> +	 * Platform NMI types (such as SERR and IOCHK) have only one
> >> +	 * handler registered per type, so there is no need to
> >> +	 * disambiguate between multiple handlers.
> >> +	 *
> >> +	 * Also, if a platform source ends up setting bit 2 in the
> >> +	 * source bitmap, the local NMI handlers would be skipped since
> >> +	 * none of them use this reserved vector.
> >> +	 *
> >> +	 * For Unknown NMIs, avoid using the source bitmap to ensure all
> >> +	 * potential handlers have a chance to claim responsibility.
> >> +	 */
> >> +	if (cpu_feature_enabled(X86_FEATURE_NMI_SOURCE) && type == NMI_LOCAL)
> >> +		source_bitmap = fred_event_data(regs);
> > 
> > 	if (cpu_feature_enabled(X86_FEATURE_NMI_SOURCE) && type == NMI_LOCAL) {
> > 		source = fred_event_data(regs);
> > 		if (source & BIT(0))
> > 			source = ~0UL;
> > 	}
> > 
> 
> 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?