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

Thomson, Jack
2026-01-16T14:33:46Z

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