Re: [PATCH] KVM: x86: SVM: Mark VMCB_LBR dirty when L1 sets DebugCtl[LBR]

[Yosry Ahmed]

On Fri, Oct 31, 2025 at 05:02:29PM -0700, Jim Mattson wrote:
> With the VMCB's LBR_VIRTUALIZATION_ENABLE bit set, the CPU will load
> the DebugCtl MSR from the VMCB's DBGCTL field at VMRUN. To ensure that
> it does not load a stale cached value, clear the VMCB's LBR clean bit
> when L1 is running and bit 0 (LBR) of the DBGCTL field is changed from
> 0 to 1. (Note that this is already handled correctly when L2 is
> running.)
> 
> There is no need to clear the clean bit in the other direction,
> because when the VMCB's DBGCTL.LBR is 0, the VMCB's
> LBR_VIRTUALIZATION_ENABLE bit will be clear, and the CPU will not
> consult the VMCB's DBGCTL field at VMRUN.

Is it worth the mental load of figuring out why we do it in
svm_enable_lbrv() but not svm_disable_lbrv()?

Maybe we can at least document it in svm_disable_lbrv() with a comment?

> 
> Fixes: 1d5a1b5860ed ("KVM: x86: nSVM: correctly virtualize LBR msrs when L2 is running")
> Reported-by: Matteo Rizzo <matteorizzo@xxxxxxxxxx>
> Reported-by: evn@xxxxxxxxxx
> Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/svm.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 153c12dbf3eb..b4e5a0684f57 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -816,6 +816,8 @@ void svm_enable_lbrv(struct kvm_vcpu *vcpu)
>  	/* Move the LBR msrs to the vmcb02 so that the guest can see them. */
>  	if (is_guest_mode(vcpu))
>  		svm_copy_lbrs(svm->vmcb, svm->vmcb01.ptr);
> +	else
> +		vmcb_mark_dirty(svm->vmcb, VMCB_LBR);
>  }
>  
>  static void svm_disable_lbrv(struct kvm_vcpu *vcpu)
> -- 
> 2.51.2.1006.ga50a493c49-goog
>