[RFC PATCH v1 11/26] KVM: arm64: Add initial support for KVM_CAP_EXIT_HYPERCALL

[Fuad Tabba <tabba@google.com>]

From: Will Deacon <will@kernel.org>

Allow the VMM to hook into and handle a subset of guest hypercalls
advertised by the host. For now, no such hypercalls exist, and so the
new capability returns 0 when queried.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/arm64/include/asm/kvm_host.h |  2 ++
 arch/arm64/kvm/arm.c              | 25 +++++++++++++++++++++++++
 arch/arm64/kvm/hypercalls.c       | 19 +++++++++++++++++++
 3 files changed, 46 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 55de71791233..f6187526685a 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -325,6 +325,8 @@ struct kvm_arch {
 	 * the associated pKVM instance in the hypervisor.
 	 */
 	struct kvm_protected_vm pkvm;
+
+	u64 hypercall_exit_enabled;
 };
 
 struct kvm_vcpu_fault_info {
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index c0e683bde111..cd6c4df27c7b 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -60,6 +60,9 @@ static bool vgic_present, kvm_arm_initialised;
 static DEFINE_PER_CPU(unsigned char, kvm_hyp_initialized);
 DEFINE_STATIC_KEY_FALSE(userspace_irqchip_in_use);
 
+/* KVM "vendor" hypercalls which may be forwarded to userspace on request. */
+#define KVM_EXIT_HYPERCALL_VALID_MASK	(0)
+
 bool is_kvm_arm_initialised(void)
 {
 	return kvm_arm_initialised;
@@ -123,6 +126,19 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		}
 		mutex_unlock(&kvm->slots_lock);
 		break;
+	case KVM_CAP_EXIT_HYPERCALL:
+		if (cap->flags)
+			return -EINVAL;
+
+		if (cap->args[0] & ~KVM_EXIT_HYPERCALL_VALID_MASK)
+			return -EINVAL;
+
+		if (cap->args[1] || cap->args[2] || cap->args[3])
+			return -EINVAL;
+
+		WRITE_ONCE(kvm->arch.hypercall_exit_enabled, cap->args[0]);
+		r = 0;
+		break;
 	default:
 		r = -EINVAL;
 		break;
@@ -334,6 +350,9 @@ static int kvm_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES:
 		r = BIT(0);
 		break;
+	case KVM_CAP_EXIT_HYPERCALL:
+		r = KVM_EXIT_HYPERCALL_VALID_MASK;
+		break;
 	default:
 		r = 0;
 	}
@@ -1071,6 +1090,12 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 		ret = kvm_handle_mmio_return(vcpu);
 		if (ret <= 0)
 			return ret;
+	} else if (run->exit_reason == KVM_EXIT_HYPERCALL) {
+		smccc_set_retval(vcpu,
+				 vcpu->run->hypercall.ret,
+				 vcpu->run->hypercall.args[0],
+				 vcpu->run->hypercall.args[1],
+				 vcpu->run->hypercall.args[2]);
 	}
 
 	vcpu_load(vcpu);
diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
index 89b5b61bc9f7..5e04be7c026a 100644
--- a/arch/arm64/kvm/hypercalls.c
+++ b/arch/arm64/kvm/hypercalls.c
@@ -132,6 +132,25 @@ static bool kvm_smccc_test_fw_bmap(struct kvm_vcpu *vcpu, u32 func_id)
 	}
 }
 
+static int __maybe_unused kvm_vcpu_exit_hcall(struct kvm_vcpu *vcpu, u32 nr, u32 nr_args)
+{
+	u64 mask = vcpu->kvm->arch.hypercall_exit_enabled;
+	u32 i;
+
+	if (nr_args > 6 || !(mask & BIT(nr))) {
+		smccc_set_retval(vcpu, SMCCC_RET_INVALID_PARAMETER, 0, 0, 0);
+		return 1;
+	}
+
+	vcpu->run->exit_reason		= KVM_EXIT_HYPERCALL;
+	vcpu->run->hypercall.nr		= nr;
+
+	for (i = 0; i < nr_args; ++i)
+		vcpu->run->hypercall.args[i] = vcpu_get_reg(vcpu, i + 1);
+
+	return 0;
+}
+
 #define SMC32_ARCH_RANGE_BEGIN	ARM_SMCCC_VERSION_FUNC_ID
 #define SMC32_ARCH_RANGE_END	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,		\
 						   ARM_SMCCC_SMC_32,		\
-- 
2.44.0.rc1.240.g4c46232300-goog