KVM: arm64: Introduce __pkvm_host_relax_guest_perms()

Introduce a new hypercall allowing the host to relax the stage-2
permissions of mappings in a non-protected guest page-table. It will be
used later once we start allowing RO memslots and dirty logging.

Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20241218194059.3670226-13-qperret@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/include/asm/kvm_asm.h

@@ -67,6 +67,7 @@ enum __kvm_host_smccc_func {
 	__KVM_HOST_SMCCC_FUNC___pkvm_host_unshare_hyp,
 	__KVM_HOST_SMCCC_FUNC___pkvm_host_share_guest,
 	__KVM_HOST_SMCCC_FUNC___pkvm_host_unshare_guest,
+	__KVM_HOST_SMCCC_FUNC___pkvm_host_relax_perms_guest,
 	__KVM_HOST_SMCCC_FUNC___kvm_adjust_pc,
 	__KVM_HOST_SMCCC_FUNC___kvm_vcpu_run,
 	__KVM_HOST_SMCCC_FUNC___kvm_flush_vm_context,

arch/arm64/kvm/hyp/include/nvhe/mem_protect.h

@@ -42,6 +42,7 @@ int __pkvm_host_unshare_ffa(u64 pfn, u64 nr_pages);
 int __pkvm_host_share_guest(u64 pfn, u64 gfn, struct pkvm_hyp_vcpu *vcpu,
 			    enum kvm_pgtable_prot prot);
 int __pkvm_host_unshare_guest(u64 gfn, struct pkvm_hyp_vm *hyp_vm);
+int __pkvm_host_relax_perms_guest(u64 gfn, struct pkvm_hyp_vcpu *vcpu, enum kvm_pgtable_prot prot);
 
 bool addr_is_memory(phys_addr_t phys);
 int host_stage2_idmap_locked(phys_addr_t addr, u64 size, enum kvm_pgtable_prot prot);

arch/arm64/kvm/hyp/nvhe/hyp-main.c

@@ -264,6 +264,25 @@ static void handle___pkvm_host_unshare_guest(struct kvm_cpu_context *host_ctxt)
 	cpu_reg(host_ctxt, 1) =  ret;
 }
 
+static void handle___pkvm_host_relax_perms_guest(struct kvm_cpu_context *host_ctxt)
+{
+	DECLARE_REG(u64, gfn, host_ctxt, 1);
+	DECLARE_REG(enum kvm_pgtable_prot, prot, host_ctxt, 2);
+	struct pkvm_hyp_vcpu *hyp_vcpu;
+	int ret = -EINVAL;
+
+	if (!is_protected_kvm_enabled())
+		goto out;
+
+	hyp_vcpu = pkvm_get_loaded_hyp_vcpu();
+	if (!hyp_vcpu || pkvm_hyp_vcpu_is_protected(hyp_vcpu))
+		goto out;
+
+	ret = __pkvm_host_relax_perms_guest(gfn, hyp_vcpu, prot);
+out:
+	cpu_reg(host_ctxt, 1) = ret;
+}
+
 static void handle___kvm_adjust_pc(struct kvm_cpu_context *host_ctxt)
 {
 	DECLARE_REG(struct kvm_vcpu *, vcpu, host_ctxt, 1);
@@ -475,6 +494,7 @@ static const hcall_t host_hcall[] = {
 	HANDLE_FUNC(__pkvm_host_unshare_hyp),
 	HANDLE_FUNC(__pkvm_host_share_guest),
 	HANDLE_FUNC(__pkvm_host_unshare_guest),
+	HANDLE_FUNC(__pkvm_host_relax_perms_guest),
 	HANDLE_FUNC(__kvm_adjust_pc),
 	HANDLE_FUNC(__kvm_vcpu_run),
 	HANDLE_FUNC(__kvm_flush_vm_context),

arch/arm64/kvm/hyp/nvhe/mem_protect.c

@@ -1488,3 +1488,26 @@ int __pkvm_host_unshare_guest(u64 gfn, struct pkvm_hyp_vm *vm)
 
 	return ret;
 }
+
+int __pkvm_host_relax_perms_guest(u64 gfn, struct pkvm_hyp_vcpu *vcpu, enum kvm_pgtable_prot prot)
+{
+	struct pkvm_hyp_vm *vm = pkvm_hyp_vcpu_to_hyp_vm(vcpu);
+	u64 ipa = hyp_pfn_to_phys(gfn);
+	u64 phys;
+	int ret;
+
+	if (prot & ~KVM_PGTABLE_PROT_RWX)
+		return -EINVAL;
+
+	host_lock_component();
+	guest_lock_component(vm);
+
+	ret = __check_host_shared_guest(vm, &phys, ipa);
+	if (!ret)
+		ret = kvm_pgtable_stage2_relax_perms(&vm->pgt, ipa, prot, 0);
+
+	guest_unlock_component(vm);
+	host_unlock_component();
+
+	return ret;
+}