linux-kernel - Re: [PATCH 5/6] KVM: SVM: Add missing save/restore handling of LBR MSRs

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Message-ID: <3759161383b53759888f64d9a03983c05026ab1c@linux.dev>
Date: Sat, 08 Nov 2025 09:08:58 +0000
From: "Yosry Ahmed" <yosry.ahmed@...ux.dev>
To: "Sean Christopherson" <seanjc@...gle.com>
Cc: "Paolo Bonzini" <pbonzini@...hat.com>, "Jim Mattson"
 <jmattson@...gle.com>, "Maxim Levitsky" <mlevitsk@...hat.com>,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 5/6] KVM: SVM: Add missing save/restore handling of LBR
 MSRs

November 7, 2025 at 4:45 PM, "Yosry Ahmed" <yosry.ahmed@...ux.dev mailto:yosry.ahmed@...ux.dev?to=%22Yosry%20Ahmed%22%20%3Cyosry.ahmed%40linux.dev%3E > wrote:
> 
> MSR_IA32_DEBUGCTLMSR and LBR MSRs are currently not enumerated by
> KVM_GET_MSR_INDEX_LIST, and LBR MSRs cannot be set with KVM_SET_MSRS. So
> save/restore is completely broken.
> 
> Fix it by adding the MSRs to msrs_to_save_base, and allowing writes to
> LBR MSRs from userspace only (as they are read-only MSRs). Additionally,
> to correctly restore L1's LBRs while L2 is running, make sure the LBRs
> are copied from the captured VMCB01 save area in svm_copy_vmrun_state().
> 
> Fixes: 24e09cbf480a ("KVM: SVM: enable LBR virtualization")
> Cc: stable@...r.kernel.org
>

Reported-by: Jim Mattson <jmattson@...gle.com>

> Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
[..]