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

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Message-ID: <7fb62597-1197-486c-bceb-0563b7a1f5a0@amd.com>
Date: Thu, 25 Sep 2025 10:46:29 +0530
From: "Upadhyay, Neeraj" <neeraj.upadhyay@....com>
To: Tom Lendacky <thomas.lendacky@....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 9/23/2025 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.

- Neeraj