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

[Sohil Mehta <sohil.mehta@intel.com>]

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.

> /* 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().