linux-kernel - Re: [PATCH v4 06/11] KVM: VMX: Expand FRED kvm entry with event data

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Message-ID: <Zr9eQdiIa-RiHGTk@google.com>
Date: Fri, 16 Aug 2024 07:12:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc: X86 Kernel <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Dave Hansen <dave.hansen@...el.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Xin Li <xin3.li@...el.com>, linux-perf-users@...r.kernel.org, 
	Peter Zijlstra <peterz@...radead.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	Tony Luck <tony.luck@...el.com>, Andy Lutomirski <luto@...nel.org>, acme@...nel.org, 
	kan.liang@...ux.intel.com, Andi Kleen <andi.kleen@...el.com>, 
	Nikolay Borisov <nik.borisov@...e.com>, Sohil Mehta <sohil.mehta@...el.com>, 
	Zeng Guang <guang.zeng@...el.com>
Subject: Re: [PATCH v4 06/11] KVM: VMX: Expand FRED kvm entry with event data

On Tue, Jul 09, 2024, Jacob Pan wrote:
> From: Zeng Guang <guang.zeng@...el.com>
> 
> According to FRED specification 10.6.2, VM exits triggered by events such as
> NMI, #DB, and #PF will have their event data stored in the exit-qualification
> field.
> 
> However, #DB and #PF are owned by the running guest, which is managed by KVM.

This is wrong.  #DB and #PF _may_ be guest #DBs/#PFs.  When host userspace is
debugging, #DBs may be the host's.  And when KVM is using shadow paging, or the
VM is running with a MAXPHYADDR smaller than the host's, #PF may be the host's
responsiblity.

> NMIs belong to the host, and the host NMI handler requires the event data
> stored in the VMCS for NMI-induced VM exits.
> 
> This patch enhances the FRED KVM entry interface to include the event data
  ^^^^^^^^^^
Documentation/process/maintainer-tip.rst

> derived from the exit qualification.

It's not "derived" from the exit qualification, it's stored there in the exact
format that's used by FRED.