[PATCH 3/6] KVM: arm64: Add pre_fault_memory implementation

Jack Thomson
2025-09-11T13:49:48Z

Add kvm_arch_vcpu_pre_fault_memory() for arm64. The implementation hands
off the stage-2 faulting logic to either gmem_abort() or
user_mem_abort().

Update __gmem_abort() and __user_mem_abort() to take the pre_fault
parameter. When passed, the paths to determine write or exec faults are
short circuited to false, as when pre-faulting, it should be treated
as a read fault.

This closely follows the implementation on x86.

Signed-off-by: Jack Thomson <jackabt@amazon.com>
---
arch/arm64/kvm/Kconfig | 1 +
arch/arm64/kvm/arm.c | 1 +
arch/arm64/kvm/mmu.c | 71 ++++++++++++++++++++++++++++++++++++------
3 files changed, 64 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index bff62e75d681..1ac0605f86cb 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -25,6 +25,7 @@ menuconfig KVM
select HAVE_KVM_CPU_RELAX_INTERCEPT
select KVM_MMIO
select KVM_GENERIC_DIRTYLOG_READ_PROTECT
+ select KVM_GENERIC_PRE_FAULT_MEMORY
select KVM_XFER_TO_GUEST_WORK
select KVM_VFIO
select HAVE_KVM_DIRTY_RING_ACQ_REL
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 888f7c7abf54..65654a742864 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -322,6 +322,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
case KVM_CAP_IRQFD_RESAMPLE:
case KVM_CAP_COUNTER_OFFSET:
case KVM_CAP_ARM_WRITABLE_IMP_ID_REGS:
+ case KVM_CAP_PRE_FAULT_MEMORY:
r = 1;
break;
case KVM_CAP_SET_GUEST_DEBUG2:
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 082e7d8ae655..002f564c6ac7 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1523,7 +1523,8 @@ static void adjust_nested_fault_perms(struct kvm_s2_trans *nested,

static int __gmem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
struct kvm_s2_trans *nested,
- struct kvm_memory_slot *memslot, bool is_perm)
+ struct kvm_memory_slot *memslot, bool is_perm,
+ bool pre_fault)
{
bool write_fault, exec_fault, writable;
enum kvm_pgtable_walk_flags flags = KVM_PGTABLE_WALK_MEMABORT_FLAGS;
@@ -1537,6 +1538,9 @@ static int __gmem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
gfn_t gfn;
int ret;

+ if (pre_fault)
+ flags |= KVM_PGTABLE_WALK_PRE_FAULT;
+
ret = prepare_mmu_memcache(vcpu, true, &memcache);
if (ret)
return ret;
@@ -1546,8 +1550,8 @@ static int __gmem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
else
gfn = fault_ipa >> PAGE_SHIFT;

- write_fault = kvm_is_write_fault(vcpu);
- exec_fault = kvm_vcpu_trap_is_exec_fault(vcpu);
+ write_fault = !pre_fault && kvm_is_write_fault(vcpu);
+ exec_fault = !pre_fault && kvm_vcpu_trap_is_exec_fault(vcpu);

VM_WARN_ON_ONCE(write_fault && exec_fault);

@@ -1599,7 +1603,7 @@ static int gmem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
struct kvm_s2_trans *nested,
struct kvm_memory_slot *memslot, bool is_perm)
{
- int ret = __gmem_abort(vcpu, fault_ipa, nested, memslot, is_perm);
+ int ret = __gmem_abort(vcpu, fault_ipa, nested, memslot, is_perm, false);
return ret != -EAGAIN ? ret : 0;
}

@@ -1607,7 +1611,7 @@ static int __user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
struct kvm_s2_trans *nested,
struct kvm_memory_slot *memslot,
long *page_size, unsigned long hva,
- bool fault_is_perm)
+ bool fault_is_perm, bool pre_fault)
{
int ret = 0;
bool topup_memcache;
@@ -1631,10 +1635,13 @@ static int __user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
vm_flags_t vm_flags;
enum kvm_pgtable_walk_flags flags = KVM_PGTABLE_WALK_MEMABORT_FLAGS;

+ if (pre_fault)
+ flags |= KVM_PGTABLE_WALK_PRE_FAULT;
+
if (fault_is_perm)
fault_granule = kvm_vcpu_trap_get_perm_fault_granule(vcpu);
- write_fault = kvm_is_write_fault(vcpu);
- exec_fault = kvm_vcpu_trap_is_exec_fault(vcpu);
+ write_fault = !pre_fault && kvm_is_write_fault(vcpu);
+ exec_fault = !pre_fault && kvm_vcpu_trap_is_exec_fault(vcpu);
VM_WARN_ON_ONCE(write_fault && exec_fault);

/*
@@ -1895,8 +1902,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
struct kvm_memory_slot *memslot, unsigned long hva,
bool fault_is_perm)
{
- int ret = __user_mem_abort(vcpu, fault_ipa, nested, memslot, NULL,
- hva, fault_is_perm);
+ int ret = __user_mem_abort(vcpu, fault_ipa, nested, memslot, NULL, hva,
+ fault_is_perm, false);
return ret != -EAGAIN ? ret : 0;
}

@@ -2468,3 +2475,49 @@ void kvm_toggle_cache(struct kvm_vcpu *vcpu, bool was_enabled)

trace_kvm_toggle_cache(*vcpu_pc(vcpu), was_enabled, now_enabled);
}
+
+long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
+ struct kvm_pre_fault_memory *range)
+{
+ int r;
+ hva_t hva;
+ phys_addr_t end;
+ long page_size;
+ struct kvm_memory_slot *memslot;
+ phys_addr_t ipa = range->gpa;
+ gfn_t gfn = gpa_to_gfn(range->gpa);
+
+ while (true) {
+ page_size = PAGE_SIZE;
+ memslot = gfn_to_memslot(vcpu->kvm, gfn);
+ if (!memslot)
+ return -ENOENT;
+
+ if (kvm_slot_has_gmem(memslot)) {
+ r = __gmem_abort(vcpu, ipa, NULL, memslot, false, true);
+ } else {
+ hva = gfn_to_hva_memslot_prot(memslot, gfn, NULL);
+ if (kvm_is_error_hva(hva))
+ return -EFAULT;
+ r = __user_mem_abort(vcpu, ipa, NULL, memslot, &page_size, hva, false,
+ true);
+ }
+
+ if (r != -EAGAIN)
+ break;
+
+ if (signal_pending(current))
+ return -EINTR;
+
+ if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu))
+ return -EIO;
+
+ cond_resched();
+ };
+
+ if (r < 0)
+ return r;
+
+ end = (range->gpa & ~(page_size - 1)) + page_size;
+ return min(range->size, end - range->gpa);
+}
--
2.43.0