Re: [PATCH v8 17/43] arm64: RME: Handle RMI_EXIT_RIPAS_CHANGE

[Steven Price <steven.price@arm.com>]

On 07/05/2025 11:42, Suzuki K Poulose wrote:
> On 16/04/2025 14:41, Steven Price wrote:
>> The guest can request that a region of it's protected address space is
>> switched between RIPAS_RAM and RIPAS_EMPTY (and back) using
>> RSI_IPA_STATE_SET. This causes a guest exit with the
>> RMI_EXIT_RIPAS_CHANGE code. We treat this as a request to convert a
>> protected region to unprotected (or back), exiting to the VMM to make
>> the necessary changes to the guest_memfd and memslot mappings. On the
>> next entry the RIPAS changes are committed by making RMI_RTT_SET_RIPAS
>> calls.
>>
>> The VMM may wish to reject the RIPAS change requested by the guest. For
>> now it can only do with by no longer scheduling the VCPU as we don't
>> currently have a usecase for returning that rejection to the guest, but
>> by postponing the RMI_RTT_SET_RIPAS changes to entry we leave the door
>> open for adding a new ioctl in the future for this purpose.
>>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> ---
>> Changes since v7:
>>   * Rework the loop in realm_set_ipa_state() to make it clear when the
>>     'next' output value of rmi_rtt_set_ripas() is used.
>> New patch for v7: The code was previously split awkwardly between two
>> other patches.
>> ---
>>   arch/arm64/kvm/rme.c | 88 ++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 88 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/rme.c b/arch/arm64/kvm/rme.c
>> index bee9dfe12e03..fe0d5b8703d2 100644
>> --- a/arch/arm64/kvm/rme.c
>> +++ b/arch/arm64/kvm/rme.c
>> @@ -624,6 +624,65 @@ void kvm_realm_unmap_range(struct kvm *kvm,
>> unsigned long start,
>>           realm_unmap_private_range(kvm, start, end);
>>   }
>>   +static int realm_set_ipa_state(struct kvm_vcpu *vcpu,
>> +                   unsigned long start,
>> +                   unsigned long end,
>> +                   unsigned long ripas,
>> +                   unsigned long *top_ipa)
>> +{
>> +    struct kvm *kvm = vcpu->kvm;
>> +    struct realm *realm = &kvm->arch.realm;
>> +    struct realm_rec *rec = &vcpu->arch.rec;
>> +    phys_addr_t rd_phys = virt_to_phys(realm->rd);
>> +    phys_addr_t rec_phys = virt_to_phys(rec->rec_page);
>> +    struct kvm_mmu_memory_cache *memcache = &vcpu->arch.mmu_page_cache;
>> +    unsigned long ipa = start;
>> +    int ret = 0;
>> +
>> +    while (ipa < end) {
>> +        unsigned long next;
>> +
>> +        ret = rmi_rtt_set_ripas(rd_phys, rec_phys, ipa, end, &next);
>> +
>> +        if (RMI_RETURN_STATUS(ret) == RMI_SUCCESS) {
>> +            ipa = next;
>> +        } else if (RMI_RETURN_STATUS(ret) == RMI_ERROR_RTT) {
>> +            int walk_level = RMI_RETURN_INDEX(ret);
>> +            int level = find_map_level(realm, ipa, end);
>> +
>> +            /*
>> +             * If the RMM walk ended early then more tables are
>> +             * needed to reach the required depth to set the RIPAS.
>> +             */
>> +            if (walk_level < level) {
>> +                ret = realm_create_rtt_levels(realm, ipa,
>> +                                  walk_level,
>> +                                  level,
>> +                                  memcache);
>> +                /* Retry with RTTs created */
> 
> minor nit: Do we need to add a comment here, saying, we stop processing
> the request if we run out of RTT pages in this go and Realm could retry
> it.

I'm not really sure this is the right place, and following Gavin's
comment I've combined this with the INIT_RIPAS code.

Also the situation at the moment isn't that we return to the guest -
kvm_complete_ripas_change() will topup the memory cache and retry until
the whole region is covered. There is an argument that perhaps it
shouldn't, but I'm not sure what a guest can usefully do beyond retry in
the case of a partial RIPAS change. In which case why have the overhead
of returning to the guest?

>> +                if (!ret)
>> +                    continue;
>> +            } else {
>> +                ret = -EINVAL;
>> +            }
>> +
>> +            break;
>> +        } else {
>> +            WARN(1, "Unexpected error in %s: %#x\n", __func__,
>> +                 ret);
>> +            ret = -ENXIO;
>> +            break;
>> +        }
> 
> minor nit: Following from Gavin's comment on another patch, could
> switch() make the above code more readable and remove the continue; ?
> 
>         switch (RMI_RETURN_STATUS(ret)) {
>         case RMI_SUCCESS:
>             ipa = next;
>             break;
>         case RMI_ERROR_RTT: {
> 
>        
>         }
>             break;
>         default:
>             WARN(..);
>             ret = -ENXIO;
>             goto out;
>         }
> 
> I am fine either way.

Since I'm combining the two functions, I've kept just the switch()
version of the code.

>> +    }
>> +
> 
> out:
> 
>> +    *top_ipa = ipa;
>> +
>> +    if (ripas == RMI_EMPTY && ipa != start)
>> +        realm_unmap_private_range(kvm, start, ipa);
>> +
>> +    return ret;
>> +}
>> +
>>   static int realm_init_ipa_state(struct realm *realm,
>>                   unsigned long ipa,
>>                   unsigned long end)
>> @@ -863,6 +922,32 @@ void kvm_destroy_realm(struct kvm *kvm)
>>       kvm_free_stage2_pgd(&kvm->arch.mmu);
>>   }
>>   +static void kvm_complete_ripas_change(struct kvm_vcpu *vcpu)
>> +{
>> +    struct kvm *kvm = vcpu->kvm;
>> +    struct realm_rec *rec = &vcpu->arch.rec;
>> +    unsigned long base = rec->run->exit.ripas_base;
>> +    unsigned long top = rec->run->exit.ripas_top;
>> +    unsigned long ripas = rec->run->exit.ripas_value;
>> +    unsigned long top_ipa;
>> +    int ret;
>> +
>> +    do {
>> +        kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_page_cache,
>> +                       kvm_mmu_cache_min_pages(vcpu->arch.hw_mmu));
>> +        write_lock(&kvm->mmu_lock);
>> +        ret = realm_set_ipa_state(vcpu, base, top, ripas, &top_ipa);
>> +        write_unlock(&kvm->mmu_lock);
>> +
>> +        if (WARN_RATELIMIT(ret && ret != -ENOMEM,
>> +                   "Unable to satisfy RIPAS_CHANGE for %#lx - %#lx,
>> ripas: %#lx\n",
>> +                   base, top, ripas))
>> +            break;
>> +
>> +        base = top_ipa;
>> +    } while (top_ipa < top);
>> +}
>> +
> 
> Rest looks good to me.

Thanks,
Steve