Re: [PATCH RFC v7 07/64] KVM: SEV: Handle KVM_HC_MAP_GPA_RANGE hypercall

[Jeremi Piotrowski]

On Wed, Dec 14, 2022 at 01:39:59PM -0600, Michael Roth wrote:
> From: Nikunj A Dadhania <nikunj@xxxxxxx>
> 
> KVM_HC_MAP_GPA_RANGE hypercall is used by the SEV guest to notify a
> change in the page encryption status to the hypervisor.
> 
> The hypercall exits to userspace with KVM_EXIT_HYPERCALL exit code,
> currently this is used for explicit memory conversion between
> shared/private for memfd based private memory.
> 
> Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx>
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> ---
>  arch/x86/kvm/x86.c  | 8 ++++++++
>  virt/kvm/kvm_main.c | 1 +
>  2 files changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index bb6adb216054..732f9cbbadb5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9649,6 +9649,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)

Couldn't find a better commit to comment on:
when the guest has the ptp-kvm module, it will issue a KVM_HC_CLOCK_PAIRING
hypercall. This will pass sev_es_validate_vmgexit validation and end up in this
function where kvm_pv_clock_pairing() is called, and that calls
kvm_write_guest(). This results in a CPU soft-lockup, at least in my testing.

Are there any emulated hypercalls that make sense for snp guests? We should
block at least the ones that definitely don't work.

Jeremi

>  		break;
>  	case KVM_HC_MAP_GPA_RANGE: {
>  		u64 gpa = a0, npages = a1, attrs = a2;
> +		struct kvm_memory_slot *slot;
>  
>  		ret = -KVM_ENOSYS;
>  		if (!(vcpu->kvm->arch.hypercall_exit_enabled & (1 << KVM_HC_MAP_GPA_RANGE)))
> @@ -9660,6 +9661,13 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  			break;
>  		}
>  
> +		slot = kvm_vcpu_gfn_to_memslot(vcpu, gpa_to_gfn(gpa));
> +		if (!vcpu->kvm->arch.upm_mode ||
> +		    !kvm_slot_can_be_private(slot)) {
> +			ret = 0;
> +			break;
> +		}
> +
>  		vcpu->run->exit_reason        = KVM_EXIT_HYPERCALL;
>  		vcpu->run->hypercall.nr       = KVM_HC_MAP_GPA_RANGE;
>  		vcpu->run->hypercall.args[0]  = gpa;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d2daa049e94a..73bf0bdedb59 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2646,6 +2646,7 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn
>  
>  	return NULL;
>  }
> +EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_memslot);
>  
>  bool kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn)
>  {
> -- 
> 2.25.1
>