Re: [PATCH v6 5/8] KVM: VMX: Extract checking of guest's DEBUGCTL into helper

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 6/11/2025 7:20 AM, Sean Christopherson wrote:
> Move VMX's logic to check DEBUGCTL values into a standalone helper so that
> the code can be used by nested VM-Enter to apply the same logic to the
> value being loaded from vmcs12.
>
> KVM needs to explicitly check vmcs12->guest_ia32_debugctl on nested
> VM-Enter, as hardware may support features that KVM does not, i.e. relying
> on hardware to detect invalid guest state will result in false negatives.
> Unfortunately, that means applying KVM's funky suppression of BTF and LBR
> to vmcs12 so as not to break existing guests.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 29 +++++++++++++++++------------
>  1 file changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ab5c742db140..358c7036272a 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2191,6 +2191,19 @@ static u64 vmx_get_supported_debugctl(struct kvm_vcpu *vcpu, bool host_initiated
>  	return debugctl;
>  }
>  
> +static bool vmx_is_valid_debugctl(struct kvm_vcpu *vcpu, u64 data,
> +				  bool host_initiated)
> +{
> +	u64 invalid;
> +
> +	invalid = data & ~vmx_get_supported_debugctl(vcpu, host_initiated);
> +	if (invalid & (DEBUGCTLMSR_BTF|DEBUGCTLMSR_LBR)) {
> +		kvm_pr_unimpl_wrmsr(vcpu, MSR_IA32_DEBUGCTLMSR, data);
> +		invalid &= ~(DEBUGCTLMSR_BTF|DEBUGCTLMSR_LBR);

nit: add spaces around "|".

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>


> +	}
> +	return !invalid;
> +}
> +
>  /*
>   * Writes msr value into the appropriate "register".
>   * Returns 0 on success, non-0 otherwise.
> @@ -2259,19 +2272,12 @@ int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		}
>  		vmcs_writel(GUEST_SYSENTER_ESP, data);
>  		break;
> -	case MSR_IA32_DEBUGCTLMSR: {
> -		u64 invalid;
> -
> -		invalid = data & ~vmx_get_supported_debugctl(vcpu, msr_info->host_initiated);
> -		if (invalid & (DEBUGCTLMSR_BTF|DEBUGCTLMSR_LBR)) {
> -			kvm_pr_unimpl_wrmsr(vcpu, msr_index, data);
> -			data &= ~(DEBUGCTLMSR_BTF|DEBUGCTLMSR_LBR);
> -			invalid &= ~(DEBUGCTLMSR_BTF|DEBUGCTLMSR_LBR);
> -		}
> -
> -		if (invalid)
> +	case MSR_IA32_DEBUGCTLMSR:
> +		if (!vmx_is_valid_debugctl(vcpu, data, msr_info->host_initiated))
>  			return 1;
>  
> +		data &= vmx_get_supported_debugctl(vcpu, msr_info->host_initiated);
> +
>  		if (is_guest_mode(vcpu) && get_vmcs12(vcpu)->vm_exit_controls &
>  						VM_EXIT_SAVE_DEBUG_CONTROLS)
>  			get_vmcs12(vcpu)->guest_ia32_debugctl = data;
> @@ -2281,7 +2287,6 @@ int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		    (data & DEBUGCTLMSR_LBR))
>  			intel_pmu_create_guest_lbr_event(vcpu);
>  		return 0;
> -	}
>  	case MSR_IA32_BNDCFGS:
>  		if (!kvm_mpx_supported() ||
>  		    (!msr_info->host_initiated &&