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

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Message-ID: <3281866f-2593-464d-a77e-5893b5e7014f@intel.com>
Date: Thu, 19 Jun 2025 15:15:21 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Xin Li <xin@...or.com>, <x86@...nel.org>, <linux-kernel@...r.kernel.org>
CC: "H . Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>, Sean Christopherson
	<seanjc@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Kan Liang
	<kan.liang@...ux.intel.com>, Tony Luck <tony.luck@...el.com>, Zhang Rui
	<rui.zhang@...el.com>, Steven Rostedt <rostedt@...dmis.org>, Andrew Cooper
	<andrew.cooper3@...rix.com>, "Kirill A . Shutemov"
	<kirill.shutemov@...ux.intel.com>, Jacob Pan <jacob.pan@...ux.microsoft.com>,
	Andi Kleen <ak@...ux.intel.com>, Kai Huang <kai.huang@...el.com>, "Sandipan
 Das" <sandipan.das@....com>, <linux-perf-users@...r.kernel.org>,
	<linux-edac@...r.kernel.org>, <kvm@...r.kernel.org>,
	<linux-pm@...r.kernel.org>, <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 02/10] x86/fred: Pass event data to the NMI entry point
 from KVM

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().