Re: [PATCH v4 4/4] KVM: x86: Use '0' for guest RIP if PMI encounters protected guest state

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2024-10-09 at 10:50 -0700, Sean Christopherson wrote:
> Explicitly return '0' for guest RIP when handling a PMI VM-Exit for a vCPU
> with protected guest state, i.e. when KVM can't read the real RIP.  While
> there is no "right" value, and profiling a protect guest is rather futile,
> returning the last known RIP is worse than returning obviously "bad" data.
> E.g. for SEV-ES+, the last known RIP will often point somewhere in the
> guest's boot flow.
> 
> Opportunistically add WARNs to effectively assert that the in_kernel() and
> get_ip() callbacks are restricted to the common PMI handler, as the return
> values for the protected guest state case are largely arbitrary, i.e. only
> make any sense whatsoever for PMIs, where the returned values have no
> functional impact and thus don't truly matter.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/x86.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 830073294640..516cf6c71567 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -13213,6 +13213,8 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>  
>  bool kvm_arch_vcpu_in_kernel(struct kvm_vcpu *vcpu)
>  {
> +	WARN_ON_ONCE(!kvm_arch_pmi_in_guest(vcpu));
> +
>  	if (vcpu->arch.guest_state_protected)
>  		return true;
>  
> @@ -13221,6 +13223,11 @@ bool kvm_arch_vcpu_in_kernel(struct kvm_vcpu *vcpu)
>  
>  unsigned long kvm_arch_vcpu_get_ip(struct kvm_vcpu *vcpu)
>  {
> +	WARN_ON_ONCE(!kvm_arch_pmi_in_guest(vcpu));
> +
> +	if (vcpu->arch.guest_state_protected)
> +		return 0;
> +
>  	return kvm_rip_read(vcpu);
>  }
>  

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
	Maxim Levitsky