[PATCH v1 06/13] KVM: arm64: Add support for KVM_MEM_USERFAULT

James Houghton
2024-12-04T19:14:22Z

Adhering to the requirements of KVM Userfault:

1. When it is toggled (either on or off), zap the second stage with
kvm_arch_flush_shadow_memslot(). This is to (1) respect
userfault-ness and (2) to reconstruct block mappings.
2. While KVM_MEM_USERFAULT is enabled, restrict new second-stage mappings
to be PAGE_SIZE, just like when dirty logging is enabled.

Signed-off-by: James Houghton <jthoughton@google.com>
---
I'm not 100% sure if kvm_arch_flush_shadow_memslot() is correct in
this case (like if the host does not have S2FWB).
---
arch/arm64/kvm/Kconfig | 1 +
arch/arm64/kvm/mmu.c | 23 ++++++++++++++++++++++-
2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index ead632ad01b4..d89b4088b580 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -38,6 +38,7 @@ menuconfig KVM
select HAVE_KVM_VCPU_RUN_PID_CHANGE
select SCHED_INFO
select GUEST_PERF_EVENTS if PERF_EVENTS
+ select HAVE_KVM_USERFAULT
help
Support hosting virtualized guest machines.

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index a71fe6f6bd90..53cee0bacb75 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1482,7 +1482,7 @@ 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_memslot_userfault(memslot)) {
force_pte = true;
vma_shift = PAGE_SHIFT;
} else {
@@ -1571,6 +1571,13 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
mmu_seq = vcpu->kvm->mmu_invalidate_seq;
mmap_read_unlock(current->mm);

+ if (kvm_gfn_userfault(kvm, memslot, gfn)) {
+ kvm_prepare_memory_fault_exit(vcpu, gfn << PAGE_SHIFT,
+ PAGE_SIZE, write_fault,
+ exec_fault, false, true);
+ return -EFAULT;
+ }
+
pfn = __kvm_faultin_pfn(memslot, gfn, write_fault ? FOLL_WRITE : 0,
&writable, &page);
if (pfn == KVM_PFN_ERR_HWPOISON) {
@@ -2062,6 +2069,20 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
enum kvm_mr_change change)
{
bool log_dirty_pages = new && new->flags & KVM_MEM_LOG_DIRTY_PAGES;
+ u32 changed_flags = (new ? new->flags : 0) ^ (old ? old->flags : 0);
+
+ /*
+ * If KVM_MEM_USERFAULT changed, drop all the stage-2 mappings so that
+ * we can (1) respect userfault-ness or (2) create block mappings.
+ */
+ if ((changed_flags & KVM_MEM_USERFAULT) && change == KVM_MR_FLAGS_ONLY)
+ kvm_arch_flush_shadow_memslot(kvm, old);
+
+ /*
+ * Nothing left to do if not toggling dirty logging.
+ */
+ if (!(changed_flags & KVM_MEM_LOG_DIRTY_PAGES))
+ return;

/*
* At this point memslot has been committed and there is an
--
2.47.0.338.g60cca15819-goog