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: <7acedeba-9c90-403c-8985-0247981bf2b5@zytor.com>
Date: Fri, 20 Jun 2025 15:51:38 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Sohil Mehta <sohil.mehta@...el.com>, Xin Li <xin@...or.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org
Cc: 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 2025-06-19 15:57, Sohil Mehta wrote:
> On 6/19/2025 3:45 PM, Xin Li wrote:
>> On 6/19/2025 3:15 PM, Sohil Mehta wrote:
>>>
>>> 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."
> 
> I am fine with this. Not *defined* removes the ambiguity.
> 

So I was thinking about this, and wonder: how expensive is it to get the 
event data exit information out of VMX? If it is not very expensive, it 
would arguably be a good thing to future-proof by fetching that 
information, even if it is currently always zero.

	-hpa