Re: [PATCH RFCv1 04/14] iommufd: Add struct iommufd_viommu and iommufd_viommu_ops

Baolu Lu
2024-05-22T09:57:35Z

On 2024/5/22 16:58, Tian, Kevin wrote:
>> From: Jason Gunthorpe<jgg@nvidia.com>
>> Sent: Tuesday, May 14, 2024 11:56 PM
>>
>> On Sun, May 12, 2024 at 08:34:02PM -0700, Nicolin Chen wrote:
>>> On Sun, May 12, 2024 at 11:03:53AM -0300, Jason Gunthorpe wrote:
>>>> On Fri, Apr 12, 2024 at 08:47:01PM -0700, Nicolin Chen wrote:
>>>>> Add a new iommufd_viommu core structure to represent a vIOMMU
>> instance in
>>>>> the user space, typically backed by a HW-accelerated feature of an
>> IOMMU,
>>>>> e.g. NVIDIA CMDQ-Virtualization (an ARM SMMUv3 extension) and
>> AMD Hardware
>>>>> Accelerated Virtualized IOMMU (vIOMMU).
>>>> I expect this will also be the only way to pass in an associated KVM,
>>>> userspace would supply the kvm when creating the viommu.
>>>>
>>>> The tricky bit of this flow is how to manage the S2. It is necessary
>>>> that the S2 be linked to the viommu:
>>>>
>>>> 1) ARM BTM requires the VMID to be shared with KVM
>>>> 2) AMD and others need the S2 translation because some of the HW
>>>> acceleration is done inside the guest address space
>>>>
>>>> I haven't looked closely at AMD but presumably the VIOMMU create will
>>>> have to install the S2 into a DID or something?
>>>>
>>>> So we need the S2 to exist before the VIOMMU is created, but the
>>>> drivers are going to need some more fixing before that will fully
>>>> work.
> Can you elaborate on this point? VIOMMU is a dummy container when
> it's created and the association to S2 comes relevant only until when
> VQUEUE is created inside and linked to a device? then there should be
> a window in between allowing the userspace to configure S2.
>
> Not saying against setting S2 up before vIOMMU creation. Just want
> to better understand the rationale here.
>
>>>> Does the nesting domain create need the viommu as well (in place of
>>>> the S2 hwpt)? That feels sort of natural.
>>> Yes, I had a similar thought initially: each viommu is backed by
>>> a nested IOMMU HW, and a special HW accelerator like VCMDQ could
>>> be treated as an extension on top of that. It might not be very
>>> straightforward like the current design having vintf<->viommu and
>>> vcmdq <-> vqueue though...
>> vqueue should be considered a sub object of the viommu and hold a
>> refcount on the viommu object for its lifetime.
>>
>>> In that case, we can then support viommu_cache_invalidate, which
>>> is quite natural for SMMUv3. Yet, I recall Kevin said that VT-d
>>> doesn't want or need that.
>> Right, Intel currently doesn't need it, but I feel like everyone will
>> need this eventually as the fast invalidation path is quite important.
>>
> yes, there is no need but I don't see any harm of preparing for such
> extension on VT-d. Logically it's clearer, e.g. if we decide to move
> device TLB invalidation to a separate uAPI then vIOMMU is certainly
> a clearer object to carry it. and hardware extensions really looks like
> optimization on software implementations.
>
> and we do need make a decision now, given if we make vIOMMU as
> a generic object for all vendors it may have potential impact on
> the user page fault support which Baolu is working on. the so-called
> fault object will be contained in vIOMMU, which is software managed
> on VT-d/SMMU but passed through on AMD. And probably we don't
> need another handle mechanism in the attach path, suppose the
> vIOMMU object already contains necessary information to find out
> iommufd_object for a reported fault.

Yes, if the vIOMMU object tracks all iommufd devices that it manages.

Best regards,
baolu