[PATCH v5 08/18] KVM: arm64: Add reclaim interface for the pKVM heap alloc

[Vincent Donnefort <vdonnefort@google.com>]

Introduce a host interface to reclaim donated memory from the pKVM heap
allocator back to the host.

It specifically provides two helpers that will make it easier to
create a shrinker for pKVM:

  pkvm_hyp_reclaimable()
  pkvm_hyp_relaim()

Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Tested-by: Fuad Tabba <fuad.tabba@linux.dev>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index de59e89318fa..fc3c52b03154 100644
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -90,6 +90,8 @@ enum __kvm_host_smccc_func {
 	__KVM_HOST_SMCCC_FUNC___vgic_v5_save_apr,
 	__KVM_HOST_SMCCC_FUNC___vgic_v5_restore_vmcr_apr,
 	__KVM_HOST_SMCCC_FUNC___pkvm_hyp_topup,
+	__KVM_HOST_SMCCC_FUNC___pkvm_hyp_reclaim,
+	__KVM_HOST_SMCCC_FUNC___pkvm_hyp_reclaimable,
 
 	MARKER(__KVM_HOST_SMCCC_FUNC_PKVM_ONLY),
 
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index 0e590261f1cc..62567560e1a5 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -851,6 +851,42 @@ static void handle___pkvm_hyp_topup(struct kvm_cpu_context *host_ctxt)
 	cpu_reg(host_ctxt, 3) = host_mc.nr_pages;
 }
 
+static void handle___pkvm_hyp_reclaim(struct kvm_cpu_context *host_ctxt)
+{
+	DECLARE_REG(enum pkvm_topup_id, id, host_ctxt, 1);
+	DECLARE_REG(unsigned long, target, host_ctxt, 2);
+	struct kvm_hyp_memcache host_mc = {};
+	int ret = 0;
+
+	switch (id) {
+	case PKVM_TOPUP_HYP_ALLOC:
+		hyp_alloc_reclaim(&host_mc, target);
+		break;
+	default:
+		ret = -EINVAL;
+	}
+
+	cpu_reg(host_ctxt, 1) = ret;
+	cpu_reg(host_ctxt, 2) = host_mc.head;
+	cpu_reg(host_ctxt, 3) = host_mc.nr_pages;
+}
+
+static void handle___pkvm_hyp_reclaimable(struct kvm_cpu_context *host_ctxt)
+{
+	DECLARE_REG(enum pkvm_topup_id, id, host_ctxt, 1);
+	unsigned long reclaimable = 0;
+
+	switch (id) {
+	case PKVM_TOPUP_HYP_ALLOC:
+		reclaimable = hyp_alloc_reclaimable();
+		break;
+	default:
+		reclaimable = 0;
+	}
+
+	cpu_reg(host_ctxt, 1) = reclaimable;
+}
+
 static void handle___tracing_load(struct kvm_cpu_context *host_ctxt)
 {
 	DECLARE_REG(unsigned long, desc_hva, host_ctxt, 1);
@@ -960,6 +996,8 @@ static const hcall_t host_hcall[] = {
 	HANDLE_FUNC(__vgic_v5_save_apr),
 	HANDLE_FUNC(__vgic_v5_restore_vmcr_apr),
 	HANDLE_FUNC(__pkvm_hyp_topup),
+	HANDLE_FUNC(__pkvm_hyp_reclaim),
+	HANDLE_FUNC(__pkvm_hyp_reclaimable),
 
 	HANDLE_FUNC(__pkvm_host_share_hyp),
 	HANDLE_FUNC(__pkvm_host_unshare_hyp),
diff --git a/arch/arm64/kvm/pkvm.c b/arch/arm64/kvm/pkvm.c
index 9cd23978481c..fec819a9dc78 100644
--- a/arch/arm64/kvm/pkvm.c
+++ b/arch/arm64/kvm/pkvm.c
@@ -115,6 +115,50 @@ static int pkvm_hyp_topup(enum pkvm_topup_id id, unsigned long nr_pages)
 	return ret;
 }
 
+static unsigned long __pkvm_hyp_reclaim(enum pkvm_topup_id id, unsigned long target)
+{
+	struct kvm_hyp_memcache mc;
+	struct arm_smccc_res res;
+	unsigned long reclaimed;
+
+	arm_smccc_1_1_hvc(KVM_HOST_SMCCC_FUNC(__pkvm_hyp_reclaim), id, target, &res);
+	if (WARN_ON_ONCE(res.a0 != SMCCC_RET_SUCCESS) || WARN_ON_ONCE(res.a1))
+		return 0;
+
+	init_hyp_memcache(&mc);
+	mc.head = res.a2;
+	mc.nr_pages = reclaimed = res.a3;
+	free_hyp_memcache(&mc);
+
+	return reclaimed;
+}
+
+static __maybe_unused unsigned long pkvm_hyp_reclaim(enum pkvm_topup_id id, unsigned long target)
+{
+	unsigned long reclaimed = 0;
+
+	while (reclaimed < target) {
+		/* Arbitrary limit to avoid blocking in EL2 for too long */
+		unsigned long r = __pkvm_hyp_reclaim(id, min(target - reclaimed, 16));
+
+		if (!r)
+			break;
+
+		reclaimed += r;
+		if (reclaimed >= target)
+			break;
+
+		cond_resched();
+	}
+
+	return reclaimed;
+}
+
+static __maybe_unused unsigned long pkvm_hyp_reclaimable(enum pkvm_topup_id id)
+{
+	return kvm_call_hyp_nvhe(__pkvm_hyp_reclaimable, id);
+}
+
 static void __pkvm_destroy_hyp_vm(struct kvm *kvm)
 {
 	if (pkvm_hyp_vm_is_created(kvm)) {
-- 
2.55.0.897.gb25b4bd76c-goog