[PATCH v6 09/14] KVM: arm64: Enable KVM_CAP_EXIT_ON_MISSING and annotate an EFAULT from stage-2 fault-handler

[Anish Moorthy <amoorthy@google.com>]

Prevent the stage-2 fault handler from faulting in pages when
KVM_MEM_EXIT_ON_MISSING is set by allowing its  __gfn_to_pfn_memslot()
calls to check the memslot flag.

To actually make that behavior useful, prepare a KVM_EXIT_MEMORY_FAULT
when the stage-2 handler cannot resolve the pfn for a fault. With
KVM_MEM_EXIT_ON_MISSING enabled this effects the delivery of stage-2
faults as vCPU exits, which userspace can attempt to resolve without
terminating the guest.

Delivering stage-2 faults to userspace in this way sidesteps the
significant scalabiliy issues associated with using userfaultfd for the
same purpose.

Signed-off-by: Anish Moorthy <amoorthy@google.com>
---
 Documentation/virt/kvm/api.rst | 2 +-
 arch/arm64/kvm/Kconfig         | 1 +
 arch/arm64/kvm/mmu.c           | 7 +++++--
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index fd87bbfbfdf2..67fcb9dbe855 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8068,7 +8068,7 @@ See KVM_EXIT_MEMORY_FAULT for more information.
 7.35 KVM_CAP_EXIT_ON_MISSING
 ----------------------------
 
-:Architectures: x86
+:Architectures: x86, arm64
 :Returns: Informational only, -EINVAL on direct KVM_ENABLE_CAP.
 
 The presence of this capability indicates that userspace may set the
diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index 1a777715199f..d6fae31f7e1a 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -43,6 +43,7 @@ menuconfig KVM
 	select GUEST_PERF_EVENTS if PERF_EVENTS
 	select INTERVAL_TREE
 	select XARRAY_MULTI
+        select HAVE_KVM_EXIT_ON_MISSING
 	help
 	  Support hosting virtualized guest machines.
 
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 13066a6fdfff..3b9fb80672ac 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1486,13 +1486,16 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	mmap_read_unlock(current->mm);
 
 	pfn = __gfn_to_pfn_memslot(memslot, gfn, false, false, NULL,
-				   write_fault, &writable, false, NULL);
+				   write_fault, &writable, true, NULL);
 	if (pfn == KVM_PFN_ERR_HWPOISON) {
 		kvm_send_hwpoison_signal(hva, vma_shift);
 		return 0;
 	}
-	if (is_error_noslot_pfn(pfn))
+	if (is_error_noslot_pfn(pfn)) {
+		kvm_prepare_memory_fault_exit(vcpu, gfn * PAGE_SIZE, PAGE_SIZE,
+					      write_fault, exec_fault, false);
 		return -EFAULT;
+	}
 
 	if (kvm_is_device_pfn(pfn)) {
 		/*
-- 
2.42.0.869.gea05f2083d-goog