[PATCH v7 09/14] KVM: arm64: Implement and advertise KVM_CAP_EXIT_ON_MISSING

[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()
call to check the memslot flag. This effects the delivery of stage-2
faults as vCPU exits (see KVM_CAP_MEMORY_FAULT_INFO), 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           | 2 +-
 3 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 7012f40332b3..01b762272b6f 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8052,7 +8052,7 @@ 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 01398d2996c7..309d8e7ebc1c 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -39,6 +39,7 @@ menuconfig KVM
 	select SCHED_INFO
 	select GUEST_PERF_EVENTS if PERF_EVENTS
 	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 5b740ddfcc8e..b0f1fef0a52c 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1487,7 +1487,7 @@ 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;
-- 
2.44.0.rc0.258.g7320e95886-goog