RE: [PATCH v7 06/10] x86/nmi: Add support to handle NMIs with source information

[Zhuo, Qiuxu]

> From: Sohil Mehta <sohil.mehta@xxxxxxxxx>
> [...]
> Activate NMI-source based filtering only for Local NMIs. While handling
> platform NMI types (such as SERR and IOCHK), do not use the source bitmap.
> They have only one handler registered per type, so there is no need to

Same as the comments for patch 6. 
Platform NMI types may have multiple handlers registered per type.

> [...]

> --- a/arch/x86/kernel/nmi.c
> +++ b/arch/x86/kernel/nmi.c
> @@ -130,6 +130,7 @@ static void nmi_check_duration(struct nmiaction
> *action, u64 duration)  static int nmi_handle(unsigned int type, struct pt_regs
> *regs)  {
>  	struct nmi_desc *desc = nmi_to_desc(type);
> +	unsigned long source_bitmap = ULONG_MAX;
>  	nmi_handler_t ehandler;
>  	struct nmiaction *a;
>  	int handled=0;
> @@ -148,16 +149,45 @@ 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

Ditto.

> +	 * 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);
> +
> +		/* Reset the bitmap if a valid source could not be identified */
> +		if (WARN_ON_ONCE(!source_bitmap) || (source_bitmap &
> BIT(NMIS_NO_SOURCE)))
> +			source_bitmap = ULONG_MAX;
> +	}
> +
>  	/*
>  	 * NMIs are edge-triggered, which means if you have enough
>  	 * of them concurrently, you can lose some because only one
>  	 * can be latched at any given time.  Walk the whole list
>  	 * to handle those situations.
> +	 *
> +	 * However, NMI-source reporting does not have this limitation.
> +	 * When NMI sources have been identified, only run the handlers
> +	 * that match the reported vectors.
>  	 */
>  	list_for_each_entry_rcu(a, &desc->head, list) {
>  		int thishandled;
>  		u64 delta;
> 
> +		if (!(source_bitmap & BIT(a->source_vector)))
> +			continue;
> +

Is it possible for the "source_bitmap" to have some non-NMIS_NO_SOURCE bit set 
while the user registers their NMI handler with the NMIS_NO_SOURCE type? 

If so, we may need to allow the NMI handler with the NMIS_NO_SOURCE type to be 
invoked unconditionally to ensure no NMIs are lost.

>  		delta = sched_clock();
>  		thishandled = a->handler(type, regs);
>  		handled += thishandled;
> --
> 2.43.0
>