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Message-ID: <bc93c396-78b1-491c-8857-41114aa585d7@gmail.com>
Date: Mon, 15 Dec 2025 11:43:06 +0000
From: "Thomson, Jack" <jackabt.amazon@...il.com>
To: Marc Zyngier <maz@...nel.org>
Cc: oliver.upton@...ux.dev, pbonzini@...hat.com, joey.gouly@....com,
suzuki.poulose@....com, yuzenghui@...wei.com, catalin.marinas@....com,
will@...nel.org, shuah@...nel.org, linux-arm-kernel@...ts.infradead.org,
kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org, isaku.yamahata@...el.com,
xmarcalx@...zon.co.uk, kalyazin@...zon.co.uk, jackabt@...zon.com
Subject: Re: [PATCH v3 1/3] KVM: arm64: Add pre_fault_memory implementation
On 24/11/2025 11:34 am, Marc Zyngier wrote:
> On Wed, 19 Nov 2025 15:49:08 +0000,
> Jack Thomson <jackabt.amazon@...il.com> wrote:
>>
>> From: Jack Thomson <jackabt@...zon.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@...zon.com>
>> ---
>> Documentation/virt/kvm/api.rst | 3 +-
>> arch/arm64/kvm/Kconfig | 1 +
>> arch/arm64/kvm/arm.c | 1 +
>> arch/arm64/kvm/mmu.c | 73 ++++++++++++++++++++++++++++++++--
>> 4 files changed, 73 insertions(+), 5 deletions(-)
>>
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index 57061fa29e6a..30872d080511 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 870953b4a8a7..88c5dc2b4ee8 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -327,6 +327,7 @@ 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;
>
> How does with pKVM, where the host is not in charge of dealing with
> stage-2?
For the pKVM case would
if (vcpu_is_protected(vcpu))
return -EPERM;
be the appropriate way to handle this?
>> break;
>> case KVM_CAP_SET_GUEST_DEBUG2:
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 7cc964af8d30..cba09168fc6d 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1599,8 +1599,8 @@ 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, long *page_size,
>
> Why is page_size a signed type? A page size is never negative.
>
>> + unsigned long hva, bool fault_is_perm)
>
> I really wish we'd stop adding parameters to this function, as it has
> long stopped being readable. It would make a lot more sense if we
> passed a descriptor for the fault, containing the ipa, hva, memslot
> and fault type.
I found a patch series which looks to address this [1]. Would you like
this fixed for this series?
[1]
https://lore.kernel.org/linux-arm-kernel/20250821210042.3451147-1-seanjc@google.com/
>> {
>> int ret = 0;
>> bool topup_memcache;
>> @@ -1878,6 +1878,9 @@ 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);
>> @@ -2080,8 +2083,8 @@ 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:
>> @@ -2457,3 +2460,65 @@ 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 ret, idx;
>> + hva_t hva;
>> + phys_addr_t end;
>> + struct kvm_memory_slot *memslot;
>> + struct kvm_vcpu_fault_info stored_fault, *fault_info;
>> +
>> + long page_size = PAGE_SIZE;
>> + phys_addr_t ipa = range->gpa;
>> + gfn_t gfn = gpa_to_gfn(range->gpa);
>
> nit: Please order this in a more readable way, preferably with long
> line first.
>
>> +
>> + idx = srcu_read_lock(&vcpu->kvm->srcu);
>
> ??? Aren't we already guaranteed to be under the SRCU read lock?
>
>> +
>> + 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;
>> + stored_fault = *fault_info;
>
> If this is a vcpu ioctl, can the fault information be actually valid
> while userspace is issuing an ioctl? Wouldn't that mean that we are
> exiting to userspace in the middle of handling an exception?
>
>> +
>> + /* Generate a synthetic abort for the pre-fault address */
>> + fault_info->esr_el2 = ESR_ELx_EC_DABT_LOW;
>> + fault_info->esr_el2 &= ~ESR_ELx_ISV;
>
> You are constructing this from scratch. How can ISV be set?
>
>> + fault_info->esr_el2 |= ESR_ELx_FSC_FAULT_L(KVM_PGTABLE_LAST_LEVEL);
>> +
>> + fault_info->hpfar_el2 = HPFAR_EL2_NS |
>> + FIELD_PREP(HPFAR_EL2_FIPA, ipa >> 12);
>> +
>> + 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;
>
> This suspiciously looks like one of the __ALIGN_KERNEL* macros.
>
>> + ret = min(range->size, end - range->gpa);
>> +
>> +out:
>> + *fault_info = stored_fault;
>> +out_unlock:
>> + srcu_read_unlock(&vcpu->kvm->srcu, idx);
>> + return ret;
>> +}
>
> Thanks,
>
> M.
>
--
Thanks,
Jack
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