[PATCH 0/6] KVM ARM64 pre_fault_memory

[PATCH 0/6] KVM ARM64 pre_fault_memory

[Jack Thomson]

From: Jack Thomson <jackabt@amazon.com>

Overview:

This patch series adds ARM64 support for the KVM_PRE_FAULT_MEMORY
feature, which was previously only available on x86 [1]. This allows
a reduction in the number of stage-2 faults during execution. This is
beneficial in post-copy migration scenarios, particularly in memory
intensive applications, where high latencies are experienced due to
the stage-2 faults when pre-populating memory via UFFD / memcpy.

Patch Overview:

 - The first patch is a preparatory refactor.

 - The second patch is adding a page walk flag for pre-faulting.

 - The third patch adds support for the KVM_PRE_FAULT_MEMORY ioctl
   on arm64.

 - The fourth patch fixes an issue with unaligned mmap allocations
   in the selftests.

 - The fifth patch updates the pre_fault_memory_test to support
   arm64.

 - The last patch extends the pre_fault_memory_test to cover
   different vm memory backings.

[1]: https://lore.kernel.org/kvm/20240710174031.312055-1-pbonzini@redhat.com

Jack Thomson (6):
  KVM: arm64: Add __gmem_abort and __user_mem_abort
  KVM: arm64: Add KVM_PGTABLE_WALK_PRE_FAULT walk flag
  KVM: arm64: Add pre_fault_memory implementation
  KVM: selftests: Fix unaligned mmap allocations
  KVM: selftests: Enable pre_fault_memory_test for arm64
  KVM: selftests: Add option for different backing in pre-fault tests

 arch/arm64/include/asm/kvm_pgtable.h          |   3 +
 arch/arm64/kvm/Kconfig                        |   1 +
 arch/arm64/kvm/arm.c                          |   1 +
 arch/arm64/kvm/hyp/pgtable.c                  |   6 +-
 arch/arm64/kvm/mmu.c                          |  97 +++++++++++++--
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 tools/testing/selftests/kvm/lib/kvm_util.c    |  12 +-
 .../selftests/kvm/pre_fault_memory_test.c     | 110 +++++++++++++-----
 8 files changed, 186 insertions(+), 45 deletions(-)


base-commit: 42188667be387867d2bf763d028654cbad046f7b
-- 
2.43.0

[PATCH 1/6] KVM: arm64: Add __gmem_abort and __user_mem_abort

[Jack Thomson]

From: Jack Thomson <jackabt@amazon.com>

Adding __gmem_abort and __user_mem_abort that preserve -EAGAIN results.
These will be used by the pre-fault implementation which needs to retry
on -EAGAIN.

Also add an optional page_size output parameter to __user_mem_abort to
return the VMA page size, which will be needed for pre-faulting.

No functional changes are intended

Signed-off-by: Jack Thomson <jackabt@amazon.com>
---
 arch/arm64/kvm/mmu.c | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index a36426ccd9b5..082e7d8ae655 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1521,9 +1521,9 @@ static void adjust_nested_fault_perms(struct kvm_s2_trans *nested,
 
 #define KVM_PGTABLE_WALK_MEMABORT_FLAGS (KVM_PGTABLE_WALK_HANDLE_FAULT | KVM_PGTABLE_WALK_SHARED)
 
-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)
+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)
 {
 	bool write_fault, exec_fault, writable;
 	enum kvm_pgtable_walk_flags flags = KVM_PGTABLE_WALK_MEMABORT_FLAGS;
@@ -1592,13 +1592,22 @@ static int gmem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	if (writable && !ret)
 		mark_page_dirty_in_slot(kvm, memslot, gfn);
 
+	return ret;
+}
+
+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);
 	return ret != -EAGAIN ? ret : 0;
 }
 
-static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
-			  struct kvm_s2_trans *nested,
-			  struct kvm_memory_slot *memslot, unsigned long hva,
-			  bool fault_is_perm)
+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)
 {
 	int ret = 0;
 	bool topup_memcache;
@@ -1871,10 +1880,23 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	kvm_release_faultin_page(kvm, page, !!ret, writable);
 	kvm_fault_unlock(kvm);
 
+	if (page_size)
+		*page_size = vma_pagesize;
+
 	/* Mark the page dirty only if the fault is handled successfully */
 	if (writable && !ret)
 		mark_page_dirty_in_slot(kvm, memslot, gfn);
 
+	return ret;
+}
+
+static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
+			  struct kvm_s2_trans *nested,
+			  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);
 	return ret != -EAGAIN ? ret : 0;
 }
 
-- 
2.43.0

Re: [PATCH 1/6] KVM: arm64: Add __gmem_abort and __user_mem_abort

[Oliver Upton]

Hi Jack,

On Thu, Sep 11, 2025 at 02:46:43PM +0100, Jack Thomson wrote:
> From: Jack Thomson <jackabt@amazon.com>
> 
> Adding __gmem_abort and __user_mem_abort that preserve -EAGAIN results.
> These will be used by the pre-fault implementation which needs to retry
> on -EAGAIN.

-EAGAIN is a pretty clear signal that another vCPU has faulted on this
memory and is in the middle of installing a mapping. Why bother with
retrying?

If we conceptually treat this thing as a synthetic stage-2 abort then it
should use the same EAGAIN handling as a literal stage-2 abort.

Thanks,
Oliver

[PATCH 2/6] KVM: arm64: Add KVM_PGTABLE_WALK_PRE_FAULT walk flag

[Jack Thomson]

From: Jack Thomson <jackabt@amazon.com>

Don't return -EAGAIN from stage2_map_walker_try_leaf during
KVM_PRE_FAULT_MEMORY.

During pre-faults, user_abort() is retried upon returning -EAGAIN,
meaning the ioctl would get stuck in an infinite loop if userspace
tries to pre-fault already existing mappings

Signed-off-by: Jack Thomson <jackabt@amazon.com>
---
 arch/arm64/include/asm/kvm_pgtable.h | 3 +++
 arch/arm64/kvm/hyp/pgtable.c         | 6 +++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 2888b5d03757..0789671d1c4f 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -296,6 +296,8 @@ typedef bool (*kvm_pgtable_force_pte_cb_t)(u64 addr, u64 end,
  * @KVM_PGTABLE_WALK_SKIP_CMO:		Visit and update table entries
  *					without Cache maintenance
  *					operations required.
+ * @KVM_PGTABLE_WALK_PRE_FAULT		Indicates the page-table walk was
+ *					invoked from a pre-fault request.
  */
 enum kvm_pgtable_walk_flags {
 	KVM_PGTABLE_WALK_LEAF			= BIT(0),
@@ -305,6 +307,7 @@ enum kvm_pgtable_walk_flags {
 	KVM_PGTABLE_WALK_HANDLE_FAULT		= BIT(4),
 	KVM_PGTABLE_WALK_SKIP_BBM_TLBI		= BIT(5),
 	KVM_PGTABLE_WALK_SKIP_CMO		= BIT(6),
+	KVM_PGTABLE_WALK_PRE_FAULT		= BIT(7),
 };
 
 struct kvm_pgtable_visit_ctx {
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index c351b4abd5db..140dccec2c5b 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -914,9 +914,13 @@ static int stage2_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx,
 	 * same mapping or only change the access permissions. Instead,
 	 * the vCPU will exit one more time from guest if still needed
 	 * and then go through the path of relaxing permissions.
+	 *
+	 * When walking in the context of a pre-fault request, if the
+	 * mapping already exists we can return 0, as there's nothing
+	 * to do.
 	 */
 	if (!stage2_pte_needs_update(ctx->old, new))
-		return -EAGAIN;
+		return (ctx->flags & KVM_PGTABLE_WALK_PRE_FAULT) ? 0 : -EAGAIN;
 
 	/* If we're only changing software bits, then store them and go! */
 	if (!kvm_pgtable_walk_shared(ctx) &&
-- 
2.43.0

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

[Jack Thomson]

From: Jack Thomson <jackabt@amazon.com>

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

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

[Oliver Upton]

On Thu, Sep 11, 2025 at 02:46:45PM +0100, Jack Thomson wrote:
> @@ -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);

I'm not a fan of this. While user_mem_abort() is already a sloppy mess,
one thing we could reliably assume is the presence of a valid fault
context. Now we need to remember to special-case our interpretation of a
fault on whether or not we're getting invoked for a pre-fault.

I'd rather see the pre-fault infrastructure compose a synthetic fault
context (HPFAR_EL2, ESR_EL2, etc.). It places the complexity where it
belongs and the rest of the abort handling code should 'just work'.

> +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();
> +	};

Why do we need another retry loop? Looks like we've already got one in
the arch-generic code.

Thanks,
Oliver

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

[Thomson, Jack]

Hi Oliver,

Thanks for reviewing!

On 11/09/2025 7:42 pm, Oliver Upton wrote:
> On Thu, Sep 11, 2025 at 02:46:45PM +0100, Jack Thomson wrote:
>> @@ -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);
> 
> I'm not a fan of this. While user_mem_abort() is already a sloppy mess,
> one thing we could reliably assume is the presence of a valid fault
> context. Now we need to remember to special-case our interpretation of a
> fault on whether or not we're getting invoked for a pre-fault.
> 
> I'd rather see the pre-fault infrastructure compose a synthetic fault
> context (HPFAR_EL2, ESR_EL2, etc.). It places the complexity where it
> belongs and the rest of the abort handling code should 'just work'.
> 

Agreed, it looks much better with the synthetic abort. Is this the
approach you had in mind?

+long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
+				    struct kvm_pre_fault_memory *range)
+{
+	int ret, idx;
+	hva_t hva;
+	phys_addr_t end;
+	u64 esr, hpfar;
+	struct kvm_memory_slot *memslot;
+	struct kvm_vcpu_fault_info *fault_info;
+
+	long page_size = PAGE_SIZE;
+	phys_addr_t ipa = range->gpa;
+	gfn_t gfn = gpa_to_gfn(range->gpa);
+
+	idx = srcu_read_lock(&vcpu->kvm->srcu);
+
+	if (ipa >= kvm_phys_size(vcpu->arch.hw_mmu)) {
+		ret = -ENOENT;
+		goto out_unlock;
+	}
+
+	memslot = gfn_to_memslot(vcpu->kvm, gfn);
+	if (!memslot) {
+		ret = -ENOENT;
+		goto out_unlock;
+	}
+
+	fault_info = &vcpu->arch.fault;
+
+	esr = fault_info->esr_el2;
+	hpfar = fault_info->hpfar_el2;
+
+	fault_info->esr_el2 = ESR_ELx_FSC_ACCESS_L(KVM_PGTABLE_LAST_LEVEL);
+	fault_info->hpfar_el2 = HPFAR_EL2_NS |
+		((ipa >> (12 - HPFAR_EL2_FIPA_SHIFT)) & HPFAR_EL2_FIPA_MASK);
+
+	if (kvm_slot_has_gmem(memslot)) {
+		ret = gmem_abort(vcpu, ipa, NULL, memslot, false);
+	} else {
+		hva = gfn_to_hva_memslot_prot(memslot, gfn, NULL);
+		if (kvm_is_error_hva(hva)) {
+			ret = -EFAULT;
+			goto out;
+		}
+		ret = user_mem_abort(vcpu, ipa, NULL, memslot, &page_size, hva,
+				     false);
+	}
+
+	if (ret < 0)
+		goto out;
+
+	end = (range->gpa & ~(page_size - 1)) + page_size;
+	ret = min(range->size, end - range->gpa);
+
+out:
+	fault_info->esr_el2 = esr;
+	fault_info->hpfar_el2 = hpfar;
+out_unlock:
+	srcu_read_unlock(&vcpu->kvm->srcu, idx);
+	return ret;
+}

>> +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();
>> +	};
> 
> Why do we need another retry loop? Looks like we've already got one in
> the arch-generic code.
>

Good point thanks, I've removed that now.

> 
> Thanks,
> Oliver

Thanks,
Jack

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

[Oliver Upton]

On Mon, Sep 29, 2025 at 02:59:35PM +0100, Thomson, Jack wrote:
> Hi Oliver,
> 
> Thanks for reviewing!
> 
> On 11/09/2025 7:42 pm, Oliver Upton wrote:
> > On Thu, Sep 11, 2025 at 02:46:45PM +0100, Jack Thomson wrote:
> > > @@ -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);
> > 
> > I'm not a fan of this. While user_mem_abort() is already a sloppy mess,
> > one thing we could reliably assume is the presence of a valid fault
> > context. Now we need to remember to special-case our interpretation of a
> > fault on whether or not we're getting invoked for a pre-fault.
> > 
> > I'd rather see the pre-fault infrastructure compose a synthetic fault
> > context (HPFAR_EL2, ESR_EL2, etc.). It places the complexity where it
> > belongs and the rest of the abort handling code should 'just work'.
> > 
> 
> Agreed, it looks much better with the synthetic abort. Is this the
> approach you had in mind?

Pretty much. Thanks for taking a moment to fiddle with it.

> +long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
> +				    struct kvm_pre_fault_memory *range)
> +{
> +	int ret, idx;
> +	hva_t hva;
> +	phys_addr_t end;
> +	u64 esr, hpfar;
> +	struct kvm_memory_slot *memslot;
> +	struct kvm_vcpu_fault_info *fault_info;
> +
> +	long page_size = PAGE_SIZE;
> +	phys_addr_t ipa = range->gpa;
> +	gfn_t gfn = gpa_to_gfn(range->gpa);
> +
> +	idx = srcu_read_lock(&vcpu->kvm->srcu);
> +
> +	if (ipa >= kvm_phys_size(vcpu->arch.hw_mmu)) {
> +		ret = -ENOENT;
> +		goto out_unlock;
> +	}
> +
> +	memslot = gfn_to_memslot(vcpu->kvm, gfn);
> +	if (!memslot) {
> +		ret = -ENOENT;
> +		goto out_unlock;
> +	}
> +
> +	fault_info = &vcpu->arch.fault;
> +
> +	esr = fault_info->esr_el2;
> +	hpfar = fault_info->hpfar_el2;

nit: Just snapshot the entire struct, makes this forward-compatible with
new fields showing up.

> +
> +	fault_info->esr_el2 = ESR_ELx_FSC_ACCESS_L(KVM_PGTABLE_LAST_LEVEL);

A translation fault would be a more accurate representation what you're
trying to do Access flag faults aren't expected in user_mem_abort() and
instead handled in handle_access_fault().

You're also missing the rest of the ESR fields that are relevant here,
such as ESR_ELx.EC which would actually indicate a data abort. I think
you'd also want to communicate this as a nISV fault (i.e.
ESR_ELx.ISV=0).

> +	fault_info->hpfar_el2 = HPFAR_EL2_NS |
> +		((ipa >> (12 - HPFAR_EL2_FIPA_SHIFT)) & HPFAR_EL2_FIPA_MASK);

FIELD_PREP()?

> +
> +	if (kvm_slot_has_gmem(memslot)) {
> +		ret = gmem_abort(vcpu, ipa, NULL, memslot, false);
> +	} else {
> +		hva = gfn_to_hva_memslot_prot(memslot, gfn, NULL);
> +		if (kvm_is_error_hva(hva)) {
> +			ret = -EFAULT;
> +			goto out;
> +		}
> +		ret = user_mem_abort(vcpu, ipa, NULL, memslot, &page_size, hva,
> +				     false);
> +	}
> +
> +	if (ret < 0)
> +		goto out;
> +
> +	end = (range->gpa & ~(page_size - 1)) + page_size;
> +	ret = min(range->size, end - range->gpa);
> +
> +out:
> +	fault_info->esr_el2 = esr;
> +	fault_info->hpfar_el2 = hpfar;
> +out_unlock:
> +	srcu_read_unlock(&vcpu->kvm->srcu, idx);
> +	return ret;
> +}

Thanks,
Oliver

[PATCH 4/6] KVM: selftests: Fix unaligned mmap allocations

[Jack Thomson]

From: Jack Thomson <jackabt@amazon.com>

When creating a VM using mmap with huge pages, and the memory amount does
not align with the underlying page size. The stored mmap_size value does
not account for the fact that mmap will automatically align the length
to a multiple of the underlying page size. During the teardown of the
test, munmap is used. However, munmap requires the length to be a
multiple of the underlying page size.

Update the vm_mem_add method to ensure the mmap_size is aligned to the
underlying page size.

Signed-off-by: Jack Thomson <jackabt@amazon.com>
---
 tools/testing/selftests/kvm/lib/kvm_util.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
index c3f5142b0a54..b106fbed999c 100644
--- a/tools/testing/selftests/kvm/lib/kvm_util.c
+++ b/tools/testing/selftests/kvm/lib/kvm_util.c
@@ -1051,7 +1051,6 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
 	/* Allocate and initialize new mem region structure. */
 	region = calloc(1, sizeof(*region));
 	TEST_ASSERT(region != NULL, "Insufficient Memory");
-	region->mmap_size = mem_size;
 
 #ifdef __s390x__
 	/* On s390x, the host address must be aligned to 1M (due to PGSTEs) */
@@ -1060,6 +1059,11 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
 	alignment = 1;
 #endif
 
+	alignment = max(backing_src_pagesz, alignment);
+	region->mmap_size = align_up(mem_size, alignment);
+
+	TEST_ASSERT_EQ(guest_paddr, align_up(guest_paddr, backing_src_pagesz));
+
 	/*
 	 * When using THP mmap is not guaranteed to returned a hugepage aligned
 	 * address so we have to pad the mmap. Padding is not needed for HugeTLB
@@ -1067,12 +1071,6 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
 	 * page size.
 	 */
 	if (src_type == VM_MEM_SRC_ANONYMOUS_THP)
-		alignment = max(backing_src_pagesz, alignment);
-
-	TEST_ASSERT_EQ(guest_paddr, align_up(guest_paddr, backing_src_pagesz));
-
-	/* Add enough memory to align up if necessary */
-	if (alignment > 1)
 		region->mmap_size += alignment;
 
 	region->fd = -1;
-- 
2.43.0

[PATCH 5/6] KVM: selftests: Enable pre_fault_memory_test for arm64

[Jack Thomson]

From: Jack Thomson <jackabt@amazon.com>

Enable the pre_fault_memory_test to run on arm64 by making it work with
different guest page sizes and testing multiple guest configurations.

Update the test_assert to compare against the UCALL_EXIT_REASON, for
portability, as arm64 exits with KVM_EXIT_MMIO while x86 uses
KVM_EXIT_IO.

Signed-off-by: Jack Thomson <jackabt@amazon.com>
---
 tools/testing/selftests/kvm/Makefile.kvm      |  1 +
 .../selftests/kvm/pre_fault_memory_test.c     | 79 ++++++++++++++-----
 2 files changed, 59 insertions(+), 21 deletions(-)

diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
index 90f03f00cb04..4db1737fad04 100644
--- a/tools/testing/selftests/kvm/Makefile.kvm
+++ b/tools/testing/selftests/kvm/Makefile.kvm
@@ -180,6 +180,7 @@ TEST_GEN_PROGS_arm64 += memslot_perf_test
 TEST_GEN_PROGS_arm64 += mmu_stress_test
 TEST_GEN_PROGS_arm64 += rseq_test
 TEST_GEN_PROGS_arm64 += steal_time
+TEST_GEN_PROGS_arm64 += pre_fault_memory_test
 
 TEST_GEN_PROGS_s390 = $(TEST_GEN_PROGS_COMMON)
 TEST_GEN_PROGS_s390 += s390/memop
diff --git a/tools/testing/selftests/kvm/pre_fault_memory_test.c b/tools/testing/selftests/kvm/pre_fault_memory_test.c
index 0350a8896a2f..ed9848a8af60 100644
--- a/tools/testing/selftests/kvm/pre_fault_memory_test.c
+++ b/tools/testing/selftests/kvm/pre_fault_memory_test.c
@@ -10,19 +10,29 @@
 #include <test_util.h>
 #include <kvm_util.h>
 #include <processor.h>
+#include <guest_modes.h>
 
 /* Arbitrarily chosen values */
-#define TEST_SIZE		(SZ_2M + PAGE_SIZE)
-#define TEST_NPAGES		(TEST_SIZE / PAGE_SIZE)
+#define TEST_BASE_SIZE		SZ_2M
 #define TEST_SLOT		10
 
+/* Storage of test info to share with guest code */
+struct test_config {
+	int page_size;
+	uint64_t test_size;
+	uint64_t test_num_pages;
+};
+
+struct test_config test_config;
+
 static void guest_code(uint64_t base_gpa)
 {
 	volatile uint64_t val __used;
+	struct test_config *config = &test_config;
 	int i;
 
-	for (i = 0; i < TEST_NPAGES; i++) {
-		uint64_t *src = (uint64_t *)(base_gpa + i * PAGE_SIZE);
+	for (i = 0; i < config->test_num_pages; i++) {
+		uint64_t *src = (uint64_t *)(base_gpa + i * config->page_size);
 
 		val = *src;
 	}
@@ -63,11 +73,17 @@ static void pre_fault_memory(struct kvm_vcpu *vcpu, u64 gpa, u64 size,
 					    "KVM_PRE_FAULT_MEMORY", ret, vcpu->vm);
 }
 
-static void __test_pre_fault_memory(unsigned long vm_type, bool private)
+struct test_params {
+	unsigned long vm_type;
+	bool private;
+};
+
+static void __test_pre_fault_memory(enum vm_guest_mode guest_mode, void *arg)
 {
+	struct test_params *p = arg;
 	const struct vm_shape shape = {
-		.mode = VM_MODE_DEFAULT,
-		.type = vm_type,
+		.mode = guest_mode,
+		.type = p->vm_type,
 	};
 	struct kvm_vcpu *vcpu;
 	struct kvm_run *run;
@@ -78,10 +94,17 @@ static void __test_pre_fault_memory(unsigned long vm_type, bool private)
 	uint64_t guest_test_virt_mem;
 	uint64_t alignment, guest_page_size;
 
+	pr_info("Testing guest mode: %s\n", vm_guest_mode_string(guest_mode));
+
 	vm = vm_create_shape_with_one_vcpu(shape, &vcpu, guest_code);
 
-	alignment = guest_page_size = vm_guest_mode_params[VM_MODE_DEFAULT].page_size;
-	guest_test_phys_mem = (vm->max_gfn - TEST_NPAGES) * guest_page_size;
+	guest_page_size = vm_guest_mode_params[guest_mode].page_size;
+
+	test_config.page_size = guest_page_size;
+	test_config.test_size = TEST_BASE_SIZE + test_config.page_size;
+	test_config.test_num_pages = vm_calc_num_guest_pages(vm->mode, test_config.test_size);
+
+	guest_test_phys_mem = (vm->max_gfn - test_config.test_num_pages) * test_config.page_size;
 #ifdef __s390x__
 	alignment = max(0x100000UL, guest_page_size);
 #else
@@ -91,22 +114,31 @@ static void __test_pre_fault_memory(unsigned long vm_type, bool private)
 	guest_test_virt_mem = guest_test_phys_mem & ((1ULL << (vm->va_bits - 1)) - 1);
 
 	vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
-				    guest_test_phys_mem, TEST_SLOT, TEST_NPAGES,
-				    private ? KVM_MEM_GUEST_MEMFD : 0);
-	virt_map(vm, guest_test_virt_mem, guest_test_phys_mem, TEST_NPAGES);
-
-	if (private)
-		vm_mem_set_private(vm, guest_test_phys_mem, TEST_SIZE);
-	pre_fault_memory(vcpu, guest_test_phys_mem, SZ_2M, 0);
-	pre_fault_memory(vcpu, guest_test_phys_mem + SZ_2M, PAGE_SIZE * 2, PAGE_SIZE);
-	pre_fault_memory(vcpu, guest_test_phys_mem + TEST_SIZE, PAGE_SIZE, PAGE_SIZE);
+				    guest_test_phys_mem, TEST_SLOT, test_config.test_num_pages,
+				    p->private ? KVM_MEM_GUEST_MEMFD : 0);
+	virt_map(vm, guest_test_virt_mem, guest_test_phys_mem, test_config.test_num_pages);
+
+	if (p->private)
+		vm_mem_set_private(vm, guest_test_phys_mem, test_config.test_size);
+	pre_fault_memory(vcpu, guest_test_phys_mem, TEST_BASE_SIZE, 0);
+	/* Test pre-faulting over an already faulted range */
+	pre_fault_memory(vcpu, guest_test_phys_mem, TEST_BASE_SIZE, 0);
+	pre_fault_memory(vcpu, guest_test_phys_mem + TEST_BASE_SIZE,
+			 test_config.page_size * 2, test_config.page_size);
+	pre_fault_memory(vcpu, guest_test_phys_mem + test_config.test_size,
+			 test_config.page_size, test_config.page_size);
 
 	vcpu_args_set(vcpu, 1, guest_test_virt_mem);
+
+	/* Export the shared variables to the guest. */
+	sync_global_to_guest(vm, test_config);
+
 	vcpu_run(vcpu);
 
 	run = vcpu->run;
-	TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
-		    "Wanted KVM_EXIT_IO, got exit reason: %u (%s)",
+	TEST_ASSERT(run->exit_reason == UCALL_EXIT_REASON,
+		    "Wanted %s, got exit reason: %u (%s)",
+		    exit_reason_str(UCALL_EXIT_REASON),
 		    run->exit_reason, exit_reason_str(run->exit_reason));
 
 	switch (get_ucall(vcpu, &uc)) {
@@ -130,7 +162,12 @@ static void test_pre_fault_memory(unsigned long vm_type, bool private)
 		return;
 	}
 
-	__test_pre_fault_memory(vm_type, private);
+	struct test_params p = {
+		.vm_type = vm_type,
+		.private = private,
+	};
+
+	for_each_guest_mode(__test_pre_fault_memory, &p);
 }
 
 int main(int argc, char *argv[])
-- 
2.43.0

[PATCH 6/6] KVM: selftests: Add option for different backing in pre-fault tests

[Jack Thomson]

From: Jack Thomson <jackabt@amazon.com>

Add a -m option to specify different memory backing types for the
pre-fault tests (e.g., anonymous, hugetlb), allowing testing of the
pre-fault functionality across different memory configurations.

Signed-off-by: Jack Thomson <jackabt@amazon.com>
---
 .../selftests/kvm/pre_fault_memory_test.c     | 31 ++++++++++++++++---
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/kvm/pre_fault_memory_test.c b/tools/testing/selftests/kvm/pre_fault_memory_test.c
index ed9848a8af60..22e2e53945d9 100644
--- a/tools/testing/selftests/kvm/pre_fault_memory_test.c
+++ b/tools/testing/selftests/kvm/pre_fault_memory_test.c
@@ -76,6 +76,7 @@ static void pre_fault_memory(struct kvm_vcpu *vcpu, u64 gpa, u64 size,
 struct test_params {
 	unsigned long vm_type;
 	bool private;
+	enum vm_mem_backing_src_type mem_backing_src;
 };
 
 static void __test_pre_fault_memory(enum vm_guest_mode guest_mode, void *arg)
@@ -94,7 +95,11 @@ static void __test_pre_fault_memory(enum vm_guest_mode guest_mode, void *arg)
 	uint64_t guest_test_virt_mem;
 	uint64_t alignment, guest_page_size;
 
+	size_t backing_src_pagesz = get_backing_src_pagesz(p->mem_backing_src);
+
 	pr_info("Testing guest mode: %s\n", vm_guest_mode_string(guest_mode));
+	pr_info("Testing memory backing src type: %s\n",
+		vm_mem_backing_src_alias(p->mem_backing_src)->name);
 
 	vm = vm_create_shape_with_one_vcpu(shape, &vcpu, guest_code);
 
@@ -110,10 +115,11 @@ static void __test_pre_fault_memory(enum vm_guest_mode guest_mode, void *arg)
 #else
 	alignment = SZ_2M;
 #endif
+	alignment = max(alignment, backing_src_pagesz);
 	guest_test_phys_mem = align_down(guest_test_phys_mem, alignment);
 	guest_test_virt_mem = guest_test_phys_mem & ((1ULL << (vm->va_bits - 1)) - 1);
 
-	vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
+	vm_userspace_mem_region_add(vm, p->mem_backing_src,
 				    guest_test_phys_mem, TEST_SLOT, test_config.test_num_pages,
 				    p->private ? KVM_MEM_GUEST_MEMFD : 0);
 	virt_map(vm, guest_test_virt_mem, guest_test_phys_mem, test_config.test_num_pages);
@@ -155,7 +161,8 @@ static void __test_pre_fault_memory(enum vm_guest_mode guest_mode, void *arg)
 	kvm_vm_free(vm);
 }
 
-static void test_pre_fault_memory(unsigned long vm_type, bool private)
+static void test_pre_fault_memory(unsigned long vm_type, enum vm_mem_backing_src_type backing_src,
+				  bool private)
 {
 	if (vm_type && !(kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(vm_type))) {
 		pr_info("Skipping tests for vm_type 0x%lx\n", vm_type);
@@ -165,6 +172,7 @@ static void test_pre_fault_memory(unsigned long vm_type, bool private)
 	struct test_params p = {
 		.vm_type = vm_type,
 		.private = private,
+		.mem_backing_src = backing_src,
 	};
 
 	for_each_guest_mode(__test_pre_fault_memory, &p);
@@ -174,10 +182,23 @@ int main(int argc, char *argv[])
 {
 	TEST_REQUIRE(kvm_check_cap(KVM_CAP_PRE_FAULT_MEMORY));
 
-	test_pre_fault_memory(0, false);
+	int opt;
+	enum vm_mem_backing_src_type backing = VM_MEM_SRC_ANONYMOUS;
+
+	while ((opt = getopt(argc, argv, "m:")) != -1) {
+		switch (opt) {
+		case 'm':
+			backing = parse_backing_src_type(optarg);
+			break;
+		default:
+			break;
+		}
+	}
+
+	test_pre_fault_memory(0, backing, false);
 #ifdef __x86_64__
-	test_pre_fault_memory(KVM_X86_SW_PROTECTED_VM, false);
-	test_pre_fault_memory(KVM_X86_SW_PROTECTED_VM, true);
+	test_pre_fault_memory(KVM_X86_SW_PROTECTED_VM, backing, false);
+	test_pre_fault_memory(KVM_X86_SW_PROTECTED_VM, backing, true);
 #endif
 	return 0;
 }
-- 
2.43.0

Re: [PATCH 0/6] KVM ARM64 pre_fault_memory

[Oliver Upton]

Hi Jack,

On Thu, Sep 11, 2025 at 02:46:42PM +0100, Jack Thomson wrote:
> From: Jack Thomson <jackabt@amazon.com>
> 
> Overview:
> 
> This patch series adds ARM64 support for the KVM_PRE_FAULT_MEMORY
> feature, which was previously only available on x86 [1]. This allows
> a reduction in the number of stage-2 faults during execution. This is
> beneficial in post-copy migration scenarios, particularly in memory
> intensive applications, where high latencies are experienced due to
> the stage-2 faults when pre-populating memory via UFFD / memcpy.

Thanks for posting the series. More of a general comment on the UAPI
documentation:

  "... However, KVM does not mark any newly created stage-2 PTE as Accessed."

This behavior is x86-specific since kvm_pgtable_stage2_map() will lay
down PTEs with the AF set. Probably shouldn't have documented the
internal state of the stage-2 in the first place but oh well, please
just update the UAPI description to make it clear this is x86-specific.

Thanks
Oliver