Re: [PATCH v7 02/10] x86/fred: Pass event data to the NMI entry point from KVM

[Xin Li]

On 6/19/2025 3:15 PM, Sohil Mehta wrote:
> On 6/18/2025 10:02 PM, Xin Li wrote:
> > > diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
> > > index 29c5c32c16c3..1c9c6e036233 100644
> > > --- a/arch/x86/entry/entry_64_fred.S
> > > +++ b/arch/x86/entry/entry_64_fred.S
> > > @@ -93,7 +93,7 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
> > >    	 * +--------+-----------------+
> > >    	 */
> > >    	push $0				/* Reserved, must be 0 */
> > > -	push $0				/* Event data, 0 for IRQ/NMI */
> > > +	push %rsi			/* Event data for NMI */
> >
> > Maybe a bit more accurate?
>
> Actually, I am wondering if it might be better to make it less precise
> than it is right now. How about
>
> /* Event data passed in by the caller */
>
> The problem with having precise comments for a generic implementation is
> that the caller might get updated, but we would forget to update this
> comment since nothing in this function needs to change.

No strong preference, I'm okay if you take this approach.

> > /* Event data, NMI-source bitmap only so far */
> >
> > >    	push %rdi			/* fred_ss handed in by the caller */
> > >    	push %rbp
> > >    	pushf
>
> ...
>
> > >    /* Must be called from noinstr code, thus __always_inline */
> > > -static __always_inline void fred_nmi_from_kvm(void)
> > > +static __always_inline void fred_nmi_from_kvm(unsigned long edata)
> > >    {
> > >    	struct fred_ss ss = {
> > >    		.ss	= __KERNEL_DS,
> > > @@ -83,7 +83,7 @@ static __always_inline void fred_nmi_from_kvm(void)
> > >    		.lm	= 1,
> > >    	};
> > >    
> > > -	asm_fred_entry_from_kvm(ss);
> > > +	asm_fred_entry_from_kvm(ss, edata);
> > >    }
> > >    
> > >    static inline void fred_irq_from_kvm(unsigned int vector)
> > > @@ -95,7 +95,8 @@ static inline void fred_irq_from_kvm(unsigned int vector)
> > >    		.lm	= 1,
> > >    	};
> > >    
> > > -	asm_fred_entry_from_kvm(ss);
> > > +	/* Event data is always zero for IRQ */
> >
> > /* Event data not used for IRQ thus 0 */
>
> Event data "not used" might imply that the architecture provides
> something, but the kernel is choosing to not use it. There is no event
> data for IRQ, right?
>
> I want to say that the event data for IRQ has to be zero until the
> architecture changes — Similar to the /* Reserved, must be 0 */ comment
> in asm_fred_entry_from_kvm().

FRED spec says:

For any other event, the event data are not currently defined and will 
be zero until they are.

So "Event data not defined for IRQ thus 0."