Re: (proposal) RE: [PATCH v7 00/16] vfio: expose virtual Shared Virtual Addressing to VMs

[Jason Wang <jasowang@redhat.com>]

On 2020/10/14 上午11:08, Tian, Kevin wrote:
>> From: Jason Wang <jasowang@redhat.com>
>> Sent: Tuesday, October 13, 2020 2:22 PM
>>
>>
>> On 2020/10/12 下午4:38, Tian, Kevin wrote:
>>>> From: Jason Wang <jasowang@redhat.com>
>>>> Sent: Monday, September 14, 2020 12:20 PM
>>>>
>>> [...]
>>>    > If it's possible, I would suggest a generic uAPI instead of a VFIO
>>>> specific one.
>>>>
>>>> Jason suggest something like /dev/sva. There will be a lot of other
>>>> subsystems that could benefit from this (e.g vDPA).
>>>>
>>>> Have you ever considered this approach?
>>>>
>>> Hi, Jason,
>>>
>>> We did some study on this approach and below is the output. It's a
>>> long writing but I didn't find a way to further abstract w/o losing
>>> necessary context. Sorry about that.
>>>
>>> Overall the real purpose of this series is to enable IOMMU nested
>>> translation capability with vSVA as one major usage, through
>>> below new uAPIs:
>>> 	1) Report/enable IOMMU nested translation capability;
>>> 	2) Allocate/free PASID;
>>> 	3) Bind/unbind guest page table;
>>> 	4) Invalidate IOMMU cache;
>>> 	5) Handle IOMMU page request/response (not in this series);
>>> 1/3/4) is the minimal set for using IOMMU nested translation, with
>>> the other two optional. For example, the guest may enable vSVA on
>>> a device without using PASID. Or, it may bind its gIOVA page table
>>> which doesn't require page fault support. Finally, all operations can
>>> be applied to either physical device or subdevice.
>>>
>>> Then we evaluated each uAPI whether generalizing it is a good thing
>>> both in concept and regarding to complexity.
>>>
>>> First, unlike other uAPIs which are all backed by iommu_ops, PASID
>>> allocation/free is through the IOASID sub-system.
>>
>> A question here, is IOASID expected to be the single management
>> interface for PASID?
> yes
>
>> (I'm asking since there're already vendor specific IDA based PASID
>> allocator e.g amdgpu_pasid_alloc())
> That comes before IOASID core was introduced. I think it should be
> changed to use the new generic interface. Jacob/Jean can better
> comment if other reason exists for this exception.


If there's no exception it should be fixed.


>
>>
>>>    From this angle
>>> we feel generalizing PASID management does make some sense.
>>> First, PASID is just a number and not related to any device before
>>> it's bound to a page table and IOMMU domain. Second, PASID is a
>>> global resource (at least on Intel VT-d),
>>
>> I think we need a definition of "global" here. It looks to me for vt-d
>> the PASID table is per device.
> PASID table is per device, thus VT-d could support per-device PASIDs
> in concept.


I think that's the requirement of PCIE spec which said PASID + RID 
identifies the process address space ID.


>   However on Intel platform we require PASIDs to be managed
> in system-wide (cross host and guest) when combining vSVA, SIOV, SR-IOV
> and ENQCMD together.


Any reason for such requirement? (I'm not familiar with ENQCMD, but my 
understanding is that vSVA, SIOV or SR-IOV doesn't have the requirement 
for system-wide PASID).


> Thus the host creates only one 'global' PASID
> namespace but do use per-device PASID table to assure isolation between
> devices on Intel platforms. But ARM does it differently as Jean explained.
> They have a global namespace for host processes on all host-owned
> devices (same as Intel), but then per-device namespace when a device
> (and its PASID table) is assigned to userspace.
>
>> Another question, is this possible to have two DMAR hardware unit(at
>> least I can see two even in my laptop). In this case, is PASID still a
>> global resource?
> yes
>
>>
>>>    while having separate VFIO/
>>> VDPA allocation interfaces may easily cause confusion in userspace,
>>> e.g. which interface to be used if both VFIO/VDPA devices exist.
>>> Moreover, an unified interface allows centralized control over how
>>> many PASIDs are allowed per process.
>>
>> Yes.
>>
>>
>>> One unclear part with this generalization is about the permission.
>>> Do we open this interface to any process or only to those which
>>> have assigned devices? If the latter, what would be the mechanism
>>> to coordinate between this new interface and specific passthrough
>>> frameworks?
>>
>> I'm not sure, but if you just want a permission, you probably can
>> introduce new capability (CAP_XXX) for this.
>>
>>
>>>    A more tricky case, vSVA support on ARM (Eric/Jean
>>> please correct me) plans to do per-device PASID namespace which
>>> is built on a bind_pasid_table iommu callback to allow guest fully
>>> manage its PASIDs on a given passthrough device.
>>
>> I see, so I think the answer is to prepare for the namespace support
>> from the start. (btw, I don't see how namespace is handled in current
>> IOASID module?)
> The PASID table is based on GPA when nested translation is enabled
> on ARM SMMU. This design implies that the guest manages PASID
> table thus PASIDs instead of going through host-side API on assigned
> device. From this angle we don't need explicit namespace in the host
> API. Just need a way to control how many PASIDs a process is allowed
> to allocate in the global namespace. btw IOASID module already has
> 'set' concept per-process and PASIDs are managed per-set. Then the
> quota control can be easily introduced in the 'set' level.
>
>>
>>>    I'm not sure
>>> how such requirement can be unified w/o involving passthrough
>>> frameworks, or whether ARM could also switch to global PASID
>>> style...
>>>
>>> Second, IOMMU nested translation is a per IOMMU domain
>>> capability. Since IOMMU domains are managed by VFIO/VDPA
>>>    (alloc/free domain, attach/detach device, set/get domain attribute,
>>> etc.), reporting/enabling the nesting capability is an natural
>>> extension to the domain uAPI of existing passthrough frameworks.
>>> Actually, VFIO already includes a nesting enable interface even
>>> before this series. So it doesn't make sense to generalize this uAPI
>>> out.
>>
>> So my understanding is that VFIO already:
>>
>> 1) use multiple fds
>> 2) separate IOMMU ops to a dedicated container fd (type1 iommu)
>> 3) provides API to associated devices/group with a container
>>
>> And all the proposal in this series is to reuse the container fd. It
>> should be possible to replace e.g type1 IOMMU with a unified module.
> yes, this is the alternative option that I raised in the last paragraph.
>
>>
>>> Then the tricky part comes with the remaining operations (3/4/5),
>>> which are all backed by iommu_ops thus effective only within an
>>> IOMMU domain. To generalize them, the first thing is to find a way
>>> to associate the sva_FD (opened through generic /dev/sva) with an
>>> IOMMU domain that is created by VFIO/VDPA. The second thing is
>>> to replicate {domain<->device/subdevice} association in /dev/sva
>>> path because some operations (e.g. page fault) is triggered/handled
>>> per device/subdevice.
>>
>> Is there any reason that the #PF can not be handled via SVA fd?
> using per-device FDs or multiplexing all fault info through one sva_FD
> is just an implementation choice. The key is to mark faults per device/
> subdevice thus anyway requires a userspace-visible handle/tag to
> represent device/subdevice and the domain/device association must
> be constructed in this new path.


I don't get why it requires a userspace-visible handle/tag. The binding 
between SVA fd and device fd could be done either explicitly or 
implicitly. So userspace know which (sub)device that this SVA fd is for.


>
>>
>>>    Therefore, /dev/sva must provide both per-
>>> domain and per-device uAPIs similar to what VFIO/VDPA already
>>> does. Moreover, mapping page fault to subdevice requires pre-
>>> registering subdevice fault data to IOMMU layer when binding
>>> guest page table, while such fault data can be only retrieved from
>>> parent driver through VFIO/VDPA.
>>>
>>> However, we failed to find a good way even at the 1st step about
>>> domain association. The iommu domains are not exposed to the
>>> userspace, and there is no 1:1 mapping between domain and device.
>>> In VFIO, all devices within the same VFIO container share the address
>>> space but they may be organized in multiple IOMMU domains based
>>> on their bus type. How (should we let) the userspace know the
>>> domain information and open an sva_FD for each domain is the main
>>> problem here.
>>
>> The SVA fd is not necessarily opened by userspace. It could be get
>> through subsystem specific uAPIs.
>>
>> E.g for vDPA if a vDPA device contains several vSVA-capable domains, we can:
>>
>> 1) introduce uAPI for userspace to know the number of vSVA-capable
>> domain
>> 2) introduce e.g VDPA_GET_SVA_FD to get the fd for each vSVA-capable
>> domain
> and also new interface to notify userspace when a domain disappears
> or a device is detached?


You need to deal with this case even in VFIO, isn't it?


>   Finally looks we are creating a completely set
> of new subsystem specific uAPIs just for generalizing another set of
> subsystem specific uAPIs. Remember after separating PASID mgmt.
> out then most of remaining vSVA uAPIs are simpler wrapper of IOMMU
> API. Replicating them is much easier logic than developing a new glue
> mechanism in each subsystem.


As discussed, the point is more than just simple generalizing. It's 
about the limitation of current uAPI. So I have the following questions:

Do we want a single PASID to be used by more than one devices? If yes, 
do we want those devices to share I/O page tables? If yes, which uAPI is 
used to program the shared I/O page tables?

Thanks


>
>>
>>> In the end we just realized that doing such generalization doesn't
>>> really lead to a clear design and instead requires tight coordination
>>> between /dev/sva and VFIO/VDPA for almost every new uAPI
>>> (especially about synchronization when the domain/device
>>> association is changed or when the device/subdevice is being reset/
>>> drained). Finally it may become a usability burden to the userspace
>>> on proper use of the two interfaces on the assigned device.
>>>
>>> Based on above analysis we feel that just generalizing PASID mgmt.
>>> might be a good thing to look at while the remaining operations are
>>> better being VFIO/VDPA specific uAPIs. anyway in concept those are
>>> just a subset of the page table management capabilities that an
>>> IOMMU domain affords. Since all other aspects of the IOMMU domain
>>> is managed by VFIO/VDPA already, continuing this path for new nesting
>>> capability sounds natural. There is another option by generalizing the
>>> entire IOMMU domain management (sort of the entire vfio_iommu_
>>> type1), but it's unclear whether such intrusive change is worthwhile
>>> (especially when VFIO/VDPA already goes different route even in legacy
>>> mapping uAPI: map/unmap vs. IOTLB).
>>>
>>> Thoughts?
>>
>> I'm ok with starting with a unified PASID management and consider the
>> unified vSVA/vIOMMU uAPI later.
>>
> Glad to see that we have consensus here. :)
>
> Thanks
> Kevin