[RFC PATCH 10/18] KVM: arm64: Add KVM Userfault support

James Houghton
2024-07-10T23:42:48Z

Support comes in three parts:
1. When KVM Userfault is enabled, only install PAGE_SIZE PTEs. This
prevents us from being able to map a userfault-enabled pfn with a
huge PTE in response to a fault on a non-userfault pfn.
2. When we get KVM_PFN_ERR_USERFAULT from __gfn_to_pfn_memslot, return a
memory fault to userspace.
3. When KVM Userfault is enabled for a particular kvm_gfn_range, unmap
it, so that we can get faults on it.

Signed-off-by: James Houghton <jthoughton@google.com>
---
arch/arm64/kvm/Kconfig | 1 +
arch/arm64/kvm/mmu.c | 36 ++++++++++++++++++++++++++++++++++--
2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index 58f09370d17e..358153d91d58 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -37,6 +37,7 @@ menuconfig KVM
select HAVE_KVM_VCPU_RUN_PID_CHANGE
select SCHED_INFO
select GUEST_PERF_EVENTS if PERF_EVENTS
+ select KVM_USERFAULT
help
Support hosting virtualized guest machines.

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 8bcab0cc3fe9..ac283e606516 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1434,7 +1434,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
* logging_active is guaranteed to never be true for VM_PFNMAP
* memslots.
*/
- if (logging_active) {
+ if (logging_active || kvm->userfault) {
force_pte = true;
vma_shift = PAGE_SHIFT;
} else {
@@ -1494,8 +1494,15 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
kvm_send_hwpoison_signal(hva, vma_shift);
return 0;
}
- if (is_error_noslot_pfn(pfn))
+ if (is_error_noslot_pfn(pfn)) {
+ if (pfn == KVM_PFN_ERR_USERFAULT)
+ kvm_prepare_memory_fault_exit(vcpu, gfn << PAGE_SHIFT,
+ PAGE_SIZE, write_fault,
+ /*exec=*/false,
+ /*private=*/false,
+ /*userfault=*/true);
return -EFAULT;
+ }

if (kvm_is_device_pfn(pfn)) {
/*
@@ -2105,3 +2112,28 @@ void kvm_toggle_cache(struct kvm_vcpu *vcpu, bool was_enabled)

trace_kvm_toggle_cache(*vcpu_pc(vcpu), was_enabled, now_enabled);
}
+
+#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
+bool kvm_arch_pre_set_memory_attributes(struct kvm *kvm,
+ struct kvm_gfn_range *range)
+{
+ unsigned long attrs = range->arg.attributes;
+
+ /*
+ * We only need to unmap if we're enabling userfault. Disabling it
+ * does not need an unmap. An unmap to get huge mappings will come
+ * later.
+ */
+ if (attrs & KVM_MEMORY_ATTRIBUTE_USERFAULT)
+ kvm_unmap_gfn_range(kvm, range);
+
+ return false;
+}
+
+bool kvm_arch_post_set_memory_attributes(struct kvm *kvm,
+ struct kvm_gfn_range *range)
+{
+ /* Nothing to do! */
+ return false;
+}
+#endif
--
2.45.2.993.g49e7a77208-goog