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: <D00BE3D4-C710-4304-85D3-B2A091E08035@zytor.com>
Date: Mon, 23 Jun 2025 09:10:07 -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 June 23, 2025 8:39:41 AM PDT, Sean Christopherson <seanjc@...gle.com> wrote:
>On Fri, Jun 20, 2025, H. Peter Anvin wrote:
>> 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.)
>
>Unless you can pinky swear that vmcs.EXIT_QUALIFICATION will provide event data
>for IRQ exits, then I'd prefer to pass '0' unconditionally.  '0' will always be
>safe, if potentially suboptimal.  But passing what could in theory be something
>other than FRED-formatted event data could lead to buggy behavior.  Per the FRED
>spec, Revision 7.0, exit-qualification doesn't hold event data for IRQ exits.
>
>  For some events for which event data is defined (see Section 5.2.1), the event
>  data is saved in the exit-qualification field. (This is done for #DB, #PF, and NMI.)

I agree, let's stick to that for now, since this is a kernel internal interface and nothing consumes it. After all, it will save a handful of cycles.

I will still check into the "pinky swear", though.