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: <36374100-0587-47f1-9319-6333f6dfe4db@zytor.com>
Date: Thu, 19 Jun 2025 15:45:21 -0700
From: Xin Li <xin@...or.com>
To: Sohil Mehta <sohil.mehta@...el.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 6/19/2025 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."