Re: [PATCH v6 03/31] KVM: SVM: Add missing save/restore handling of LBR MSRs

[Yosry Ahmed]

On Tue, Feb 24, 2026 at 2:34 PM Yosry Ahmed <yosry@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> Reported-by: Jim Mattson <jmattson@xxxxxxxxxx>
> Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/nested.c |  3 +++
>  arch/x86/kvm/svm/svm.c    | 24 ++++++++++++++++++++++++
>  arch/x86/kvm/x86.c        |  3 +++
>  3 files changed, 30 insertions(+)
>
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index f7d5db0af69ac..52d8536845927 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1100,6 +1100,9 @@ void svm_copy_vmrun_state(struct vmcb_save_area *to_save,
>                 to_save->isst_addr = from_save->isst_addr;
>                 to_save->ssp = from_save->ssp;
>         }
> +
> +       if (lbrv)
> +               svm_copy_lbrs(to_save, from_save);

We need to clear reserved bits here, similar to
nested_vmcb02_prepare_save(), as this is stuff by userspace.

>  }