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: <a74a3da8-e030-4e6a-b3d8-b13d5f10e4e4@intel.com>
Date: Fri, 13 Jun 2025 08:20:32 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: <x86@...nel.org>, <linux-kernel@...r.kernel.org>, Xin Li <xin@...or.com>,
	"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>, 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 6/12/2025 5:18 PM, Sean Christopherson wrote:

>> Read the VMCS exit qualification unconditionally since almost all
>> upcoming CPUs are expected to enable FRED and NMI-source together. In
>> the rare case that NMI-source isn't enabled, the extra VMREAD would be
>> harmless since the exit qualification is expected to be zero.
> 
> Nit, instead of "is expected to be zero", something like this 
> 
>   harmless since the exit qualification is architecturally guaranteed to be
>   zero on CPUs that don't support NMI-source reporting.  Per the SDM's
>   "Exit qualification" subsection of "Basic VM-Exit Information":
> 
>       For all other VM exits, this field is cleared.
> --
> 

Looks good. Clarifying it explicitly reduces ambiguity.

If this patchset gets applied directly, I am hoping the clarification
can be included while applying. If we end up doing another version, I'll
add it to the log.

> to make it very explicit that reading the exit qualification on older CPUs is 100%
> safe, e.g. even on non-FRED CPUs (see https://lore.kernel.org/all/aBUiwLV4ZY2HdRbz@google.com).
> 

That's interesting. Thanks for the link.

>> Suggested-by: Sean Christopherson <seanjc@...gle.com>
>> Originally-by: Zeng Guang <guang.zeng@...el.com>
>> Signed-off-by: Sohil Mehta <sohil.mehta@...el.com>
>> ---
> 
> Acked-by: Sean Christopherson <seanjc@...gle.com>

Thank you!