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

[H. Peter Anvin]

On May 8, 2025 5:15:44 AM PDT, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>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?

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