Re: [PATCH v18] arm64: mm: Handle Granule Protection Faults (GPFs)

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Hi Catalin

On 22/09/2026 15:49, Catalin Marinas wrote:
> On Tue, Sep 22, 2026 at 02:21:13PM +0100, Suzuki K Poulose wrote:
>> I had another look and we could handle this via kvm_fault_is_gmem_abort()
>> see in arch/arm64/kvm/mmu.c:
>>
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 87e49251e0447..af5a4bf961aae 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1731,6 +1731,9 @@ static int gmem_abort(const struct kvm_s2_fault_desc
>> *s2fd)
>>          gfn_t gfn;
>>          int ret;
>>
>> +       if (!kvm_slot_has_gmem(s2fd->memslot))
>> +               return -EINVAL;
> 
> I wonder whether we should add a KVM_BUG_ON() here. With the rest of the
> changes, we should never get in this situation. Well, to be revisited
> for private devices.
> 
> Also maybe move it to the caller, kvm_vm_mem_abort(), and not change
> kvm_fault_is_gmem_abort(). Something like:
> 
> 	if (private_ipa_fault(kvm, s2fd->fault_ipa) &&
> 	    KVM_BUG_ON(!kvm_slot_has_gmem(s2fd->memslot), kvm))
> 		return -EIO;
> 
> To me it makes more sense for gmem_abort() to be called only *if* it's a
> gmem slot. So any inconsistency, avoiding user_mem_abort() for private
> memory, should be done in the caller. I assume the caller will also have
> to route the private device path as well rather than rely on
> gmem_abort().
> 
>> +
>>          if (!perm_fault) {
>>                  memcache = get_mmu_memcache(vcpu);
>>                  ret = topup_mmu_memcache(vcpu, memcache);
>> @@ -2277,10 +2280,12 @@ static bool private_ipa_fault(struct kvm *kvm,
>> phys_addr_t fault_ipa);
>>   static bool kvm_fault_is_gmem_abort(struct kvm *kvm,
>>                                      const struct kvm_s2_fault_desc *s2fd)
>>   {
>> -       if (!kvm_slot_has_gmem(s2fd->memslot))
>> -               return false;
>>          if (kvm_memslot_is_gmem_only(s2fd->memslot))
>>                  return true;
>> +       /*
>> +        * For Realms, all private faults must be backed by GMEM.
>> +        * TODO: Handle Trusted device private memory mappings.
>> +        */
>>          if (private_ipa_fault(kvm, s2fd->fault_ipa))
>>                  return true;
>>          return false;
>>
>>
>> Also, I have the following hunk for preventing memslot modifications.
>> I will add this to v20 integration branch, which is almost ready ;-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 582b48e34486b..87e49251e0447 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -2783,6 +2783,18 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>>          }
>>   }
>>
>> +static bool kvm_prevents_memslot_change(struct kvm *kvm, enum kvm_mr_change change)
>> +{
>> +       /* Cannot modify memslots once a pVM has run or Realm created */
>> +       if (change != KVM_MR_DELETE && change != KVM_MR_MOVE)
>> +               return false;
>> +
>> +       if ((kvm_vm_is_protected_pkvm(kvm) && pkvm_hyp_vm_is_created(kvm)) ||
>> +            kvm_realm_is_created(kvm))
>> +               return true;
>> +       return false;
>> +}
>> +

This needs to be tweaked for Realm to support non-secure device 
assignment. Aneesh reports that the Device assignment fails now,
because the Device BAR reset deletes the memory slot and re-registers
it, which the above change prevents.

I will modify that to
1. Prevent "Guest-memfd" backed memory slot deletion. Makes sure that
    nothing can replace a private memory slot.
2. Allow non-Guest-memfd backed memory slots to be created
   after the Realm is created. We anyways prevent "private" memory
   to be mapped from a non-Guest-memfd memslot.

Suzuki