[RFC PATCH v2 3/6] KVM: arm64: Refactor hvc filtering to support different actions

[Oliver Upton <oliver.upton@linux.dev>]

KVM presently allows userspace to filter guest hypercalls with bitmaps
expressed via pseudo-firmware registers. These bitmaps have a narrow
scope and, of course, can only allow/deny a particular call. A
subsequent change to KVM will introduce a generalized UAPI for filtering
hypercalls, allowing functions to be forwarded to userspace.

Refactor the existing hypercall filtering logic to make room for more
than two actions. While at it, generalize the function names around
SMCCC as it is the basis for the upcoming UAPI.

No functional change intended.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/include/uapi/asm/kvm.h |  9 +++++++++
 arch/arm64/kvm/hypercalls.c       | 19 +++++++++++++++----
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index a7a857f1784d..e298574a45ea 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -468,6 +468,15 @@ enum {
 /* run->fail_entry.hardware_entry_failure_reason codes. */
 #define KVM_EXIT_FAIL_ENTRY_CPU_UNSUPPORTED	(1ULL << 0)
 
+enum kvm_smccc_filter_action {
+	KVM_SMCCC_FILTER_ALLOW = 0,
+	KVM_SMCCC_FILTER_DENY,
+
+#ifdef __KERNEL__
+	NR_SMCCC_FILTER_ACTIONS
+#endif
+};
+
 #endif
 
 #endif /* __ARM_KVM_H__ */
diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
index c9f401fa01a9..980546b295b3 100644
--- a/arch/arm64/kvm/hypercalls.c
+++ b/arch/arm64/kvm/hypercalls.c
@@ -65,7 +65,7 @@ static void kvm_ptp_get_time(struct kvm_vcpu *vcpu, u64 *val)
 	val[3] = lower_32_bits(cycles);
 }
 
-static bool kvm_hvc_call_default_allowed(u32 func_id)
+static bool kvm_smccc_default_call(u32 func_id)
 {
 	switch (func_id) {
 	/*
@@ -93,7 +93,7 @@ static bool kvm_hvc_call_default_allowed(u32 func_id)
 	}
 }
 
-static bool kvm_hvc_call_allowed(struct kvm_vcpu *vcpu, u32 func_id)
+static bool kvm_smccc_test_fw_bmap(struct kvm_vcpu *vcpu, u32 func_id)
 {
 	struct kvm_smccc_features *smccc_feat = &vcpu->kvm->arch.smccc_feat;
 
@@ -117,19 +117,30 @@ static bool kvm_hvc_call_allowed(struct kvm_vcpu *vcpu, u32 func_id)
 		return test_bit(KVM_REG_ARM_VENDOR_HYP_BIT_PTP,
 				&smccc_feat->vendor_hyp_bmap);
 	default:
-		return kvm_hvc_call_default_allowed(func_id);
+		return false;
 	}
 }
 
+static u8 kvm_hvc_get_action(struct kvm_vcpu *vcpu, u32 func_id)
+{
+	if (kvm_smccc_default_call(func_id) ||
+	    kvm_smccc_test_fw_bmap(vcpu, func_id))
+		return KVM_SMCCC_FILTER_ALLOW;
+
+	return KVM_SMCCC_FILTER_DENY;
+}
+
 int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 {
 	struct kvm_smccc_features *smccc_feat = &vcpu->kvm->arch.smccc_feat;
 	u32 func_id = smccc_get_function(vcpu);
 	u64 val[4] = {SMCCC_RET_NOT_SUPPORTED};
 	u32 feature;
+	u8 action;
 	gpa_t gpa;
 
-	if (!kvm_hvc_call_allowed(vcpu, func_id))
+	action = kvm_hvc_get_action(vcpu, func_id);
+	if (action == KVM_SMCCC_FILTER_DENY)
 		goto out;
 
 	switch (func_id) {
-- 
2.39.1.581.gbfd45094c4-goog


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