Re: [PATCH v4 05/12] vfio/iommufd: Introduce auto domain creation

[Joao Martins <joao.m.martins@oracle.com>]

On 16/07/2024 10:39, Cédric Le Goater wrote:
> On 7/12/24 13:46, Joao Martins wrote:
>> There's generally two modes of operation for IOMMUFD:
>>
>> * The simple user API which intends to perform relatively simple things
>> with IOMMUs e.g. DPDK. It generally creates an IOAS and attach to VFIO
>> and mainly performs IOAS_MAP and UNMAP.
>>
>> * The native IOMMUFD API where you have fine grained control of the
>> IOMMU domain and model it accordingly. This is where most new feature
>> are being steered to.
>>
>> For dirty tracking 2) 
> 
> I suppose 1) and 2) are the bullets above ?
> 
yeah

>> is required, as it needs to ensure that
>> the stage-2/parent IOMMU domain will only attach devices
>> that support dirty tracking (so far it is all homogeneous in x86, likely
>> not the case for smmuv3). Such invariant on dirty tracking provides a
>> useful guarantee to VMMs that will refuse incompatible device
>> attachments for IOMMU domains.
>>
>> Dirty tracking insurance is enforced via HWPT_ALLOC, which is
>> responsible for creating an IOMMU domain. This is contrast to the
>> 'simple API' where the IOMMU domain is created by IOMMUFD automatically
>> when it attaches to VFIO (usually referred as autodomains) but it has
>> the needed handling for mdevs.
>>
>> To support dirty tracking with the advanced IOMMUFD API, it needs
>> similar logic, where IOMMU domains are created and devices attached to
>> compatible domains. Essentially mimmicing kernel
> 
> mimmicing -> mimicking, I think.
> 

Ack

>> iommufd_device_auto_get_domain(). With mdevs given there's no IOMMU domain
>> it falls back to IOAS attach.
>>
>> The auto domain logic allows different IOMMU domains to be created when
>> DMA dirty tracking is not desired (and VF can provide it), and others where
>> it is. Here is not used in this way here given how VFIODevice migration
>> state is initialized after the device attachment. But such mixed mode of
>> IOMMU dirty tracking + device dirty tracking is an improvement that can
>> be added on. Keep the 'all of nothing' of type1 approach that we have
>> been using so far between container vs device dirty tracking.
>>
>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> 
> Reviewed-by: Cédric Le Goater <clg@redhat.com>
> 
> This needs feedback from IOMMUFD experts also.

I take it that by IOMMUFD experts you the ones more familiar with Qemu side
(Zhenzhong and/or Yi)

	Joao