Re: [PATCH v4 1/3] KVM: arm64: Add pre_fault_memory implementation

["Thomson, Jack" <jackabt.amazon@gmail.com>]

Hey Marc,

Thanks for the review.

On 15/01/2026 9:51 am, Marc Zyngier wrote:
> [+ Vladimir, who was also looking at this patch]
> 
> On Tue, 13 Jan 2026 15:26:40 +0000,
> Jack Thomson <jackabt.amazon@gmail.com> wrote:
>>
>> 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().
>>
>> Add an optional page_size output parameter to user_mem_abort() to
>> return the VMA page size, which is needed when pre-faulting.
>>
>> Update the documentation to clarify x86 specific behaviour.
>>
>> Signed-off-by: Jack Thomson <jackabt@amazon.com>
>> ---
>>   Documentation/virt/kvm/api.rst |  3 +-
>>   arch/arm64/kvm/Kconfig         |  1 +
>>   arch/arm64/kvm/arm.c           |  1 +
>>   arch/arm64/kvm/mmu.c           | 79 ++++++++++++++++++++++++++++++++--
>>   4 files changed, 79 insertions(+), 5 deletions(-)
>>
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index 01a3abef8abb..44cfd9e736bb 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -6493,7 +6493,8 @@ Errors:
>>   KVM_PRE_FAULT_MEMORY populates KVM's stage-2 page tables used to map memory
>>   for the current vCPU state.  KVM maps memory as if the vCPU generated a
>>   stage-2 read page fault, e.g. faults in memory as needed, but doesn't break
>> -CoW.  However, KVM does not mark any newly created stage-2 PTE as Accessed.
>> +CoW.  However, on x86, KVM does not mark any newly created stage-2 PTE as
>> +Accessed.
>>   
>>   In the case of confidential VM types where there is an initial set up of
>>   private guest memory before the guest is 'finalized'/measured, this ioctl
>> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
>> index 4f803fd1c99a..6872aaabe16c 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 VIRT_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 4f80da0c0d1d..19bac68f737f 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -332,6 +332,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>>   	case KVM_CAP_COUNTER_OFFSET:
>>   	case KVM_CAP_ARM_WRITABLE_IMP_ID_REGS:
>>   	case KVM_CAP_ARM_SEA_TO_USER:
>> +	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 48d7c372a4cd..499b131f794e 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1642,8 +1642,8 @@ static int gmem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   
>>   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)
>> +			  struct kvm_memory_slot *memslot, unsigned long *page_size,
>> +			  unsigned long hva, bool fault_is_perm)
>>   {
>>   	int ret = 0;
>>   	bool topup_memcache;
>> @@ -1923,6 +1923,9 @@ 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);
>> @@ -2196,8 +2199,8 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
>>   		ret = gmem_abort(vcpu, fault_ipa, nested, memslot,
>>   				 esr_fsc_is_permission_fault(esr));
>>   	else
>> -		ret = user_mem_abort(vcpu, fault_ipa, nested, memslot, hva,
>> -				     esr_fsc_is_permission_fault(esr));
>> +		ret = user_mem_abort(vcpu, fault_ipa, nested, memslot, NULL,
>> +				     hva, esr_fsc_is_permission_fault(esr));
>>   	if (ret == 0)
>>   		ret = 1;
>>   out:
>> @@ -2573,3 +2576,71 @@ 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)
>> +{
>> +	struct kvm_vcpu_fault_info *fault_info = &vcpu->arch.fault;
>> +	struct kvm_s2_trans nested_trans, *nested = NULL;
>> +	unsigned long page_size = PAGE_SIZE;
>> +	struct kvm_memory_slot *memslot;
>> +	phys_addr_t ipa = range->gpa;
>> +	phys_addr_t end;
>> +	hva_t hva;
>> +	gfn_t gfn;
>> +	int ret;
>> +
>> +	if (vcpu_is_protected(vcpu))
>> +		return -EOPNOTSUPP;
> 
> This feels pretty odd. If you have advertised the capability, then
> saying "not supported" at this stage is not on.
> 

Thanks good point, I think I can actually just drop this completely since
kvm_pvm_ext_allowed() would already exclude this as a capacility.

>> +
>> +	/*
>> +	 * We may prefault on a shadow stage 2 page table if we are
>> +	 * running a nested guest.  In this case, we have to resolve the L2
>> +	 * IPA to the L1 IPA first, before knowing what kind of memory should
>> +	 * back the L1 IPA.
>> +	 *
>> +	 * If the shadow stage 2 page table walk faults, then we return
>> +	 * -EFAULT
>> +	 */
>> +	if (kvm_is_nested_s2_mmu(vcpu->kvm, vcpu->arch.hw_mmu) &&
>> +	    vcpu->arch.hw_mmu->nested_stage2_enabled) {
>> +		ret = kvm_walk_nested_s2(vcpu, ipa, &nested_trans);
>> +		if (ret)
>> +			return -EFAULT;
> 
> And then what? Userspace is completely screwed here, with no way to
> make any forward progress, because the L1 is in charge of that S2, and
> L1 is not running. What's the outcome? Light a candle and pray?
> 
> Also, the IPA you are passing as a parameter means absolutely nothing
> in the context of L2. Userspace doesn't have the faintest clue about
> the memory map presented to L2, as that's L1 business. L1 can
> absolutely present to L2 a memory map that doesn't have a single
> address in common with its own.
> 
> So this really doesn't work at all.
> 
Would just returning -EOPNOTSUPP in this case like:

   if (kvm_is_nested_s2_mmu(vcpu->kvm, vcpu->arch.hw_mmu) &&
       vcpu->arch.hw_mmu->nested_stage2_enabled)
     return -EOPNOTSUPP;

be the best way to continue for now?
>> +
>> +		ipa = kvm_s2_trans_output(&nested_trans);
>> +		nested = &nested_trans;
>> +	}
>> +
>> +	if (ipa >= kvm_phys_size(vcpu->arch.hw_mmu))
>> +		return -ENOENT;
>> +
>> +	/* Generate a synthetic abort for the pre-fault address */
>> +	fault_info->esr_el2 = (ESR_ELx_EC_DABT_LOW << ESR_ELx_EC_SHIFT) |
>> +		ESR_ELx_FSC_FAULT_L(KVM_PGTABLE_LAST_LEVEL);
> 
> Why level 3? You must present a fault that matches the level at which
> the emulated fault would actually occur, because the rest of the
> infrastructure relies on that (at least on the permission path, and
> more to come).
> 

Ack, thanks I was relying on the fact `fault_is_perm` was hardcoded to
false. I'll replace with something like:

   pgt = vcpu->arch.hw_mmu->pgt;
   ret = kvm_pgtable_get_leaf(pgt, gpa, &pte, &level);
   if (ret)
     return ret;

   fault_info->esr_el2 = (ESR_ELx_EC_DABT_LOW << ESR_ELx_EC_SHIFT) |
     ESR_ELx_FSC_FAULT_L(level);
   fault_info->hpfar_el2 = HPFAR_EL2_NS |
     FIELD_PREP(HPFAR_EL2_FIPA, gpa >> 12);

> Taking a step back on all this, 90% of the problems are there because
> you are trying to support prefaulting a guest that is already running.
> If you limited this to actually *pre*-faulting the guest, it would be
> the easiest thing ever, and wouldn't suffer from any of the above (you
> can't be in a nested context if you haven't run).
> 
> What prevents you from doing so? I'm perfectly happy to make this a
> separate API if this contradicts other implementations. Or are you
> relying on other side effects of the "already running" state?

We would need this to work on an already running guest.

> 
> Thanks,
> 
> 	M.

Thanks again for taking a look!

-- 
Thanks,
Jack