KVM: s390: Add helper functions for fault handling

Add some helper functions for handling multiple guest faults at the
same time.

This will be needed for VSIE, where a nested guest access also needs to
access all the page tables that map it.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

arch/s390/include/asm/kvm_host.h

@@ -442,6 +442,7 @@ struct kvm_vcpu_arch {
 	bool acrs_loaded;
 	struct kvm_s390_pv_vcpu pv;
 	union diag318_info diag318_info;
+	void *mc; /* Placeholder */
 };
 
 struct kvm_vm_stat {

arch/s390/kvm/Makefile

@@ -9,7 +9,7 @@ ccflags-y := -Ivirt/kvm -Iarch/s390/kvm
 
 kvm-y += kvm-s390.o intercept.o interrupt.o priv.o sigp.o
 kvm-y += diag.o gaccess.o guestdbg.o vsie.o pv.o gmap-vsie.o
-kvm-y += dat.o gmap.o
+kvm-y += dat.o gmap.o faultin.o
 
 kvm-$(CONFIG_VFIO_PCI_ZDEV_KVM) += pci.o
 obj-$(CONFIG_KVM) += kvm.o

arch/s390/kvm/faultin.c

@@ -0,0 +1,148 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  KVM guest fault handling.
+ *
+ *    Copyright IBM Corp. 2025
+ *    Author(s): Claudio Imbrenda <imbrenda@linux.ibm.com>
+ */
+#include <linux/kvm_types.h>
+#include <linux/kvm_host.h>
+
+#include "gmap.h"
+#include "trace.h"
+#include "faultin.h"
+
+bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu);
+
+/*
+ * kvm_s390_faultin_gfn() - handle a dat fault.
+ * @vcpu: The vCPU whose gmap is to be fixed up, or NULL if operating on the VM.
+ * @kvm: The VM whose gmap is to be fixed up, or NULL if operating on a vCPU.
+ * @f: The guest fault that needs to be resolved.
+ *
+ * Return:
+ * * 0 on success
+ * * < 0 in case of error
+ * * > 0 in case of guest exceptions
+ *
+ * Context:
+ * * The mm lock must not be held before calling
+ * * kvm->srcu must be held
+ * * may sleep
+ */
+int kvm_s390_faultin_gfn(struct kvm_vcpu *vcpu, struct kvm *kvm, struct guest_fault *f)
+{
+	struct kvm_s390_mmu_cache *local_mc __free(kvm_s390_mmu_cache) = NULL;
+	struct kvm_s390_mmu_cache *mc = NULL;
+	struct kvm_memory_slot *slot;
+	unsigned long inv_seq;
+	int foll, rc = 0;
+
+	foll = f->write_attempt ? FOLL_WRITE : 0;
+	foll |= f->attempt_pfault ? FOLL_NOWAIT : 0;
+
+	if (vcpu) {
+		kvm = vcpu->kvm;
+		mc = vcpu->arch.mc;
+	}
+
+	lockdep_assert_held(&kvm->srcu);
+
+	scoped_guard(read_lock, &kvm->mmu_lock) {
+		if (gmap_try_fixup_minor(kvm->arch.gmap, f) == 0)
+			return 0;
+	}
+
+	while (1) {
+		f->valid = false;
+		inv_seq = kvm->mmu_invalidate_seq;
+		/* Pairs with the smp_wmb() in kvm_mmu_invalidate_end(). */
+		smp_rmb();
+
+		if (vcpu)
+			slot = kvm_vcpu_gfn_to_memslot(vcpu, f->gfn);
+		else
+			slot = gfn_to_memslot(kvm, f->gfn);
+		f->pfn = __kvm_faultin_pfn(slot, f->gfn, foll, &f->writable, &f->page);
+
+		/* Needs I/O, try to setup async pfault (only possible with FOLL_NOWAIT). */
+		if (f->pfn == KVM_PFN_ERR_NEEDS_IO) {
+			if (unlikely(!f->attempt_pfault))
+				return -EAGAIN;
+			if (unlikely(!vcpu))
+				return -EINVAL;
+			trace_kvm_s390_major_guest_pfault(vcpu);
+			if (kvm_arch_setup_async_pf(vcpu))
+				return 0;
+			vcpu->stat.pfault_sync++;
+			/* Could not setup async pfault, try again synchronously. */
+			foll &= ~FOLL_NOWAIT;
+			f->pfn = __kvm_faultin_pfn(slot, f->gfn, foll, &f->writable, &f->page);
+		}
+
+		/* Access outside memory, addressing exception. */
+		if (is_noslot_pfn(f->pfn))
+			return PGM_ADDRESSING;
+		/* Signal pending: try again. */
+		if (f->pfn == KVM_PFN_ERR_SIGPENDING)
+			return -EAGAIN;
+		/* Check if it's read-only memory; don't try to actually handle that case. */
+		if (f->pfn == KVM_PFN_ERR_RO_FAULT)
+			return -EOPNOTSUPP;
+		/* Any other error. */
+		if (is_error_pfn(f->pfn))
+			return -EFAULT;
+
+		if (!mc) {
+			local_mc = kvm_s390_new_mmu_cache();
+			if (!local_mc)
+				return -ENOMEM;
+			mc = local_mc;
+		}
+
+		/* Loop, will automatically release the faulted page. */
+		if (mmu_invalidate_retry_gfn_unsafe(kvm, inv_seq, f->gfn)) {
+			kvm_release_faultin_page(kvm, f->page, true, false);
+			continue;
+		}
+
+		scoped_guard(read_lock, &kvm->mmu_lock) {
+			if (!mmu_invalidate_retry_gfn(kvm, inv_seq, f->gfn)) {
+				f->valid = true;
+				rc = gmap_link(mc, kvm->arch.gmap, f);
+				kvm_release_faultin_page(kvm, f->page, !!rc, f->write_attempt);
+				f->page = NULL;
+			}
+		}
+		kvm_release_faultin_page(kvm, f->page, true, false);
+
+		if (rc == -ENOMEM) {
+			rc = kvm_s390_mmu_cache_topup(mc);
+			if (rc)
+				return rc;
+		} else if (rc != -EAGAIN) {
+			return rc;
+		}
+	}
+}
+
+int kvm_s390_get_guest_page(struct kvm *kvm, struct guest_fault *f, gfn_t gfn, bool w)
+{
+	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+	int foll = w ? FOLL_WRITE : 0;
+
+	f->write_attempt = w;
+	f->gfn = gfn;
+	f->pfn = __kvm_faultin_pfn(slot, gfn, foll, &f->writable, &f->page);
+	if (is_noslot_pfn(f->pfn))
+		return PGM_ADDRESSING;
+	if (is_sigpending_pfn(f->pfn))
+		return -EINTR;
+	if (f->pfn == KVM_PFN_ERR_NEEDS_IO)
+		return -EAGAIN;
+	if (is_error_pfn(f->pfn))
+		return -EFAULT;
+
+	f->valid = true;
+	return 0;
+}

arch/s390/kvm/faultin.h

@@ -0,0 +1,92 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ *  KVM guest fault handling.
+ *
+ *    Copyright IBM Corp. 2025
+ *    Author(s): Claudio Imbrenda <imbrenda@linux.ibm.com>
+ */
+
+#ifndef __KVM_S390_FAULTIN_H
+#define __KVM_S390_FAULTIN_H
+
+#include <linux/kvm_host.h>
+
+#include "dat.h"
+
+int kvm_s390_faultin_gfn(struct kvm_vcpu *vcpu, struct kvm *kvm, struct guest_fault *f);
+int kvm_s390_get_guest_page(struct kvm *kvm, struct guest_fault *f, gfn_t gfn, bool w);
+
+static inline int kvm_s390_faultin_gfn_simple(struct kvm_vcpu *vcpu, struct kvm *kvm,
+					      gfn_t gfn, bool wr)
+{
+	struct guest_fault f = { .gfn = gfn, .write_attempt = wr, };
+
+	return kvm_s390_faultin_gfn(vcpu, kvm, &f);
+}
+
+static inline int kvm_s390_get_guest_page_and_read_gpa(struct kvm *kvm, struct guest_fault *f,
+						       gpa_t gaddr, unsigned long *val)
+{
+	int rc;
+
+	rc = kvm_s390_get_guest_page(kvm, f, gpa_to_gfn(gaddr), false);
+	if (rc)
+		return rc;
+
+	*val = *(unsigned long *)phys_to_virt(pfn_to_phys(f->pfn) | offset_in_page(gaddr));
+
+	return 0;
+}
+
+static inline void kvm_s390_release_multiple(struct kvm *kvm, struct guest_fault *guest_faults,
+					     int n, bool ignore)
+{
+	int i;
+
+	for (i = 0; i < n; i++) {
+		kvm_release_faultin_page(kvm, guest_faults[i].page, ignore,
+					 guest_faults[i].write_attempt);
+		guest_faults[i].page = NULL;
+	}
+}
+
+static inline bool kvm_s390_multiple_faults_need_retry(struct kvm *kvm, unsigned long seq,
+						       struct guest_fault *guest_faults, int n,
+						       bool unsafe)
+{
+	int i;
+
+	for (i = 0; i < n; i++) {
+		if (!guest_faults[i].valid)
+			continue;
+		if (unsafe && mmu_invalidate_retry_gfn_unsafe(kvm, seq, guest_faults[i].gfn))
+			return true;
+		if (!unsafe && mmu_invalidate_retry_gfn(kvm, seq, guest_faults[i].gfn))
+			return true;
+	}
+	return false;
+}
+
+static inline int kvm_s390_get_guest_pages(struct kvm *kvm, struct guest_fault *guest_faults,
+					   gfn_t start, int n_pages, bool write_attempt)
+{
+	int i, rc;
+
+	for (i = 0; i < n_pages; i++) {
+		rc = kvm_s390_get_guest_page(kvm, guest_faults + i, start + i, write_attempt);
+		if (rc)
+			break;
+	}
+	return rc;
+}
+
+#define kvm_s390_release_faultin_array(kvm, array, ignore) \
+	kvm_s390_release_multiple(kvm, array, ARRAY_SIZE(array), ignore)
+
+#define kvm_s390_array_needs_retry_unsafe(kvm, seq, array) \
+	kvm_s390_multiple_faults_need_retry(kvm, seq, array, ARRAY_SIZE(array), true)
+
+#define kvm_s390_array_needs_retry_safe(kvm, seq, array) \
+	kvm_s390_multiple_faults_need_retry(kvm, seq, array, ARRAY_SIZE(array), false)
+
+#endif /* __KVM_S390_FAULTIN_H */

arch/s390/kvm/kvm-s390.c

@@ -4637,7 +4637,7 @@ bool kvm_arch_can_dequeue_async_page_present(struct kvm_vcpu *vcpu)
 	return true;
 }
 
-static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu)
+bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu)
 {
 	hva_t hva;
 	struct kvm_arch_async_pf arch;

arch/s390/kvm/kvm-s390.h

@@ -470,6 +470,8 @@ static inline int kvm_s390_handle_dat_fault(struct kvm_vcpu *vcpu, gpa_t gaddr,
 	return __kvm_s390_handle_dat_fault(vcpu, gpa_to_gfn(gaddr), gaddr, flags);
 }
 
+bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu);
+
 /* implemented in diag.c */
 int kvm_s390_handle_diag(struct kvm_vcpu *vcpu);