linux-kernel - Re: [PATCH v3 20/27] KVM: x86: Allow WRMSRNS to be advertised to guests

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Message-ID: <Z73kmRwZFJJAVkiZ@google.com>
Date: Tue, 25 Feb 2025 07:41:13 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: "Xin Li (Intel)" <xin@...or.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, pbonzini@...hat.com, corbet@....net, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, luto@...nel.org, 
	peterz@...radead.org, andrew.cooper3@...rix.com
Subject: Re: [PATCH v3 20/27] KVM: x86: Allow WRMSRNS to be advertised to guests

On Mon, Sep 30, 2024, Xin Li (Intel) wrote:
> From: Xin Li <xin3.li@...el.com>
> 
> Allow WRMSRNS to be advertised to guests.

The shortlog and this sentence are incorrect.  Assuming there are no controls for
WRMSRNS, then KVM isn't allowing anything.  Userspace can advertise WRMSRNS support
whenever it wants, and the guest can cleanly execute WRMSRNS regardless of whether
or not it's advertised in CPUID.  KVM is simply advertising support to userspace.

> WRMSRNS behaves exactly like WRMSR with the only difference being

Nope, not the only difference.

  WRMSR and WRMSRNS use the same basic exit reason (see Appendix C). For WRMSR,
  the exit qualification is 0, while for WRMSRNS it is 1.

And the whole reaosn I went spelunking was to verify that WRMSRNS honors all MSR
exiting controls and generates the same exits.  That information needs to be
explicitly stated.

I'll rewrite the shortlog and changelog when applying.