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: <1713a225-44e0-4018-bf5f-64ffd7746167@zytor.com>
Date: Fri, 20 Jun 2025 16:22:12 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Sohil Mehta <sohil.mehta@...el.com>, Xin Li <xin@...or.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        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>,
        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 2025-06-20 16:18, Sean Christopherson wrote:
>>
>> 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.
> 
> It's trivially easy to do in KVM, and the cost of the VMREAD should be less than
> 20 cycles.  So quite cheap in the grand scheme.  If VMREAD is more costly than
> that, then we have bigger problems :-)
> 

LOL. Since it is up to you, Paulo, etc. to decide how to do the 
tradeoffs formaintainability, debuggability and performance in KVM I am 
guessing this is a vote in favor? (You can always take it out if it is a 
performance problem, until such time that the kernel itself starts 
consuming this information for reasons currently unknown.)

	-hpa