Re: [PATCH 5/6] iommu: document missing def_domain_type return

Robin Murphy
2024-12-11T18:42:57Z

On 10/12/2024 10:06 pm, Matthew Rosato wrote:
> On 12/10/24 1:42 PM, Robin Murphy wrote:
>> On 10/12/2024 4:26 pm, Matthew Rosato wrote:
>>> On 12/9/24 9:57 PM, Baolu Lu wrote:
>>>> On 12/10/24 03:24, Matthew Rosato wrote:
>>>>> In addition to IOMMU_DOMAIN_DMA, def_domain_type can also return
>>>>> IOMMU_DOMAIN_DMA_FQ when applicable, else flush queues will never be
>>>>> used.
>>>>>
>>>>> Signed-off-by: Matthew Rosato<mjrosato@linux.ibm.com>
>>>>> ---
>>>>>    include/linux/iommu.h | 1 +
>>>>>    1 file changed, 1 insertion(+)
>>>>>
>>>>> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
>>>>> index 05279109c732..d0da1918d2de 100644
>>>>> --- a/include/linux/iommu.h
>>>>> +++ b/include/linux/iommu.h
>>>>> @@ -585,6 +585,7 @@ iommu_copy_struct_from_full_user_array(void *kdst, size_t kdst_entry_size,
>>>>>     * @def_domain_type: device default domain type, return value:
>>>>>     *        - IOMMU_DOMAIN_IDENTITY: must use an identity domain
>>>>>     *        - IOMMU_DOMAIN_DMA: must use a dma domain
>>>>> + *              - IOMMU_DOMAIN_DMA_FQ: dma domain with batch invalidation
>>>>
>>>> In which case must an iommu driver return IOMMU_DOMAIN_DMA_FQ?
>>>>
>>>> The flush queue is a policy of "when and how to synchronize the IOTLB"
>>>> in dma-iommu.c. The iommu driver actually has no need to understand such
>>>> policy.
>>>
>>> If you look ahead to the next patch where I implement def_domain_type for s390, I found that if I only ever return IOMMU_DOMAIN_DMA from ops->def_domain_type then when go through iommu_dma_init_domain() we will never call iommu_dma_init_fq() regardless of IOMMU_CAP_DEFERRED_FLUSH because of the if (domain->type == IOMMU_DOMAIN_DMA_FQ) check.  So something isn't right here.
>>
>> Conceptually I don't think it ever makes sense for a driver to *require* a device to use deferred invalidation. Furthermore it's been the whole design for a while now that drivers should never see nor have to acknowledge IOMMU_DOMAIN_DMA_FQ, it's now just an internal type which exists largely for the sake of making the sysfs interface work really neatly. Also beware that a major reason for overriding iommu_def_domain_type with a paging domain is for untrusted devices, so massaging the result based on iommu_dma_strict is still not necessarily appropriate anyway.
>>
>> It appears the real underlying issue is that, like everyone else in the same situation, you're doing def_domain_type wrong. If and when you can't support IOMMU_DOMAIN_IDENTITY, the expectation is that you make __iommu_alloc_identity_domain() fail, such that if iommu_def_domain_type is then ever set to passthrough, iommu_group_alloc_default_domain() falls back to IOMMU_DOMAIN_DMA by itself, and the user gets told they did a silly thing.
>
> OK, I almost see where this all fits to throw out def_domain_type for this series... but looking at __iommu_alloc_identity_domain, the preferred approach is using a static identity domain which turns __iommu_alloc_identity_domain into a nofail case once you define the identity_domain:
>
> if (ops->identity_domain)
> return ops->identity_domain;
>
> So it seems to me to be an all-or-nothing thing, whereas what I'm trying to achieve is a device-based decision on whether the group is allowed to use that identity domain. Which reminds me that this is ultimately why I ended up looking into def_domain_type in the first place.

Well, yeah, the static domain is very much designed for the typical case
where it *is* unconditionally available. Simple options right now would
be to have two sets of ops and select them per-instance, or if it's a
system-wide property perhaps have non-const ops and populate/clear
identity_domain as appropriate, or possibly even just stick with the
ops->domain_alloc(IOMMU_DOMAIN_IDENTITY) path (perhaps eventually
evolving into a specialised domain_alloc_identity op to also become
per-device?). What we should not have is mechanisms like the
def_domain_type hack where something claims to be available, but then
some other thing tries to say "oh but you mustn't touch that".

Thanks,
Robin.

> If I need __iommu_alloc_identity_domain to fail, I guess what I'm looking to do boils down to something like...
>
> if (ops->identity_domain) {
> if (!ops->allow_identity || ops->allow_identity(dev))
> return ops->identity_domain;
> else
> return ERR_PTR(-EOPNOTSUPP);
> }
>
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