linux-kernel - Re: [RFC PATCH v2 05/17] KVM: SVM: Do not intercept SECURE_AVIC

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Message-ID: <93d32f0e-5c66-44bc-83af-cb3e7bd40fc1@amd.com>
Date: Thu, 25 Sep 2025 08:54:46 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: "Upadhyay, Neeraj" <neeraj.upadhyay@....com>, kvm@...r.kernel.org,
 seanjc@...gle.com, pbonzini@...hat.com
Cc: linux-kernel@...r.kernel.org, nikunj@....com, Santosh.Shukla@....com,
 Vasant.Hegde@....com, Suravee.Suthikulpanit@....com, bp@...en8.de,
 David.Kaplan@....com, huibo.wang@....com, naveen.rao@....com,
 tiala@...rosoft.com
Subject: Re: [RFC PATCH v2 05/17] KVM: SVM: Do not intercept
 SECURE_AVIC_CONTROL MSR for SAVIC guests

On 9/25/25 00:16, Upadhyay, Neeraj wrote:
> 
> 
> On 9/23/2025 7:25 PM, Tom Lendacky wrote:
>> On 9/23/25 00:03, Neeraj Upadhyay wrote:
>>> Disable interception for SECURE_AVIC_CONTROL MSR for Secure AVIC
>>> enabled guests. The SECURE_AVIC_CONTROL MSR holds the GPA of the
>>> guest APIC backing page and bitfields to control enablement of Secure
>>> AVIC and whether the guest allows NMIs to be injected by the hypervisor.
>>> This MSR is populated by the guest and can be read by the guest to get
>>> the GPA of the APIC backing page. The MSR can only be accessed in Secure
>>> AVIC mode; accessing it when not in Secure AVIC mode results in #GP. So,
>>> KVM should not intercept it.
>>
>> The reason KVM should not intercept the MSR access is that the guest
>> would not be able to actually set the MSR if it is intercepted.
>>
> 
> Yes, something like below looks ok?
> 
> Disable interception for SECURE_AVIC_CONTROL MSR for Secure AVIC
> enabled guests. The SECURE_AVIC_CONTROL MSR holds the GPA of the
> guest APIC backing page and bitfields to control enablement of Secure
> AVIC and whether the guest allows NMIs to be injected by the hypervisor.
> This MSR is populated by the guest and can be read by the guest to get
> the GPA of the APIC backing page. This MSR is only accessible by the
> guest when the Secure AVIC feature is active; any other access attempt
> will result in a #GP fault. So, KVM should not intercept access to this
> MSR, as doing so prevents the guest from successfully reading/writing its
> configuration and enabling the feature.

It's probably more info than is really needed. Just saying something like
the following should be enough (feel free to improve on this):

Disable interception of the SECURE_AVIC_CONTROL MSR for Secure AVIC
enabled guests. The SECURE_AVIC_CONTROL MSR is used by the guest to
configure and enable Secure AVIC. In order for the guest to be able to
successfully do this, the MSR access must not be intercepted.

Thanks,
Tom


> 
> 
> 
> - Neeraj
>