KVM: arm64: Track KVM IOCTLs and their associated KVM caps

Track KVM IOCTLs (VM IOCTLs for now), and the associated KVM capability
that enables that IOCTL. Add a function that performs the lookup.

This will be used by CoCo VM Hypervisors (e.g., pKVM) to determine
whether a particular KVM IOCTL is allowed for its VMs.

Suggested-by: Oliver Upton <oupton@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
[maz: don't expose KVM_CAP_BASIC to userspace, and rely on NR_VCPUS
      as a proxy for this]
Link: https://patch.msgid.link/20251211104710.151771-8-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Fuad Tabba

arch/arm64/include/asm/kvm_host.h

@@ -1655,4 +1655,6 @@ static __always_inline enum fgt_group_id __fgt_reg_to_group_id(enum vcpu_sysreg
 		p;							\
 	})
 
+long kvm_get_cap_for_kvm_ioctl(unsigned int ioctl, long *ext);
+
 #endif /* __ARM64_KVM_HOST_H__ */

arch/arm64/kvm/arm.c

@@ -58,6 +58,51 @@ enum kvm_wfx_trap_policy {
 static enum kvm_wfx_trap_policy kvm_wfi_trap_policy __read_mostly = KVM_WFX_NOTRAP_SINGLE_TASK;
 static enum kvm_wfx_trap_policy kvm_wfe_trap_policy __read_mostly = KVM_WFX_NOTRAP_SINGLE_TASK;
 
+/*
+ * Tracks KVM IOCTLs and their associated KVM capabilities.
+ */
+struct kvm_ioctl_cap_map {
+	unsigned int ioctl;
+	long ext;
+};
+
+/* Make KVM_CAP_NR_VCPUS the reference for features we always supported */
+#define KVM_CAP_ARM_BASIC	KVM_CAP_NR_VCPUS
+
+/*
+ * Sorted by ioctl to allow for potential binary search,
+ * though linear scan is sufficient for this size.
+ */
+static const struct kvm_ioctl_cap_map vm_ioctl_caps[] = {
+	{ KVM_CREATE_IRQCHIP, KVM_CAP_IRQCHIP },
+	{ KVM_ARM_SET_DEVICE_ADDR, KVM_CAP_ARM_SET_DEVICE_ADDR },
+	{ KVM_ARM_MTE_COPY_TAGS, KVM_CAP_ARM_MTE },
+	{ KVM_SET_DEVICE_ATTR, KVM_CAP_DEVICE_CTRL },
+	{ KVM_GET_DEVICE_ATTR, KVM_CAP_DEVICE_CTRL },
+	{ KVM_HAS_DEVICE_ATTR, KVM_CAP_DEVICE_CTRL },
+	{ KVM_ARM_SET_COUNTER_OFFSET, KVM_CAP_COUNTER_OFFSET },
+	{ KVM_ARM_GET_REG_WRITABLE_MASKS, KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES },
+	{ KVM_ARM_PREFERRED_TARGET, KVM_CAP_ARM_BASIC },
+};
+
+/*
+ * Set *ext to the capability.
+ * Return 0 if found, or -EINVAL if no IOCTL matches.
+ */
+long kvm_get_cap_for_kvm_ioctl(unsigned int ioctl, long *ext)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(vm_ioctl_caps); i++) {
+		if (vm_ioctl_caps[i].ioctl == ioctl) {
+			*ext = vm_ioctl_caps[i].ext;
+			return 0;
+		}
+	}
+
+	return -EINVAL;
+}
+
 DECLARE_KVM_HYP_PER_CPU(unsigned long, kvm_hyp_vector);
 
 DEFINE_PER_CPU(unsigned long, kvm_arm_hyp_stack_base);