[PATCH v6 07/14] KVM: x86: Enable KVM_CAP_EXIT_ON_MISSING and annotate EFAULTs 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 returns EFAULT, e.g. when it cannot resolve the
pfn. 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/x86/kvm/Kconfig           | 1 +
 arch/x86/kvm/mmu/mmu.c         | 8 ++++++--
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 1457865f6e98..fd87bbfbfdf2 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: None
+:Architectures: x86
 :Returns: Informational only, -EINVAL on direct KVM_ENABLE_CAP.
 
 The presence of this capability indicates that userspace may set the
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index c1716e83d176..97b16be349a2 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -49,6 +49,7 @@ config KVM
 	select INTERVAL_TREE
 	select HAVE_KVM_PM_NOTIFIER if PM
 	select KVM_GENERIC_HARDWARE_ENABLING
+        select HAVE_KVM_EXIT_ON_MISSING
 	help
 	  Support hosting fully virtualized guest machines using hardware
 	  virtualization extensions.  You will need a fairly recent
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index b1e5e42bdeb4..bc978260d2be 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3309,6 +3309,10 @@ static int kvm_handle_error_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fa
 		return RET_PF_RETRY;
 	}
 
+	WARN_ON_ONCE(fault->goal_level != PG_LEVEL_4K);
+
+	kvm_prepare_memory_fault_exit(vcpu, gfn_to_gpa(fault->gfn), PAGE_SIZE,
+				      fault->write, fault->exec, fault->is_private);
 	return -EFAULT;
 }
 
@@ -4375,7 +4379,7 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 	async = false;
 	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, false, &async,
 					  fault->write, &fault->map_writable,
-					  false, &fault->hva);
+					  true, &fault->hva);
 	if (!async)
 		return RET_PF_CONTINUE; /* *pfn has correct page already */
 
@@ -4397,7 +4401,7 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 	 */
 	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, true, NULL,
 					  fault->write, &fault->map_writable,
-					  false, &fault->hva);
+					  true, &fault->hva);
 	return RET_PF_CONTINUE;
 }
 
-- 
2.42.0.869.gea05f2083d-goog