Re: [RFC PATCH 00/12] Private MMIO support for private assigned dev

[Zhi Wang <zhiw@nvidia.com>]

On 15.5.2025 13.29, Alexey Kardashevskiy wrote:
> 
> 
> On 13/5/25 20:03, Zhi Wang wrote:
>> On Mon, 12 May 2025 11:06:17 -0300
>> Jason Gunthorpe <jgg@nvidia.com> wrote:
>>
>>> On Mon, May 12, 2025 at 07:30:21PM +1000, Alexey Kardashevskiy wrote:
>>>
>>>>>> I'm surprised by this.. iommufd shouldn't be doing PCI stuff,
>>>>>> it is just about managing the translation control of the device.
>>>>>
>>>>> I have a little difficulty to understand. Is TSM bind PCI stuff?
>>>>> To me it is. Host sends PCI TDISP messages via PCI DOE to put the
>>>>> device in TDISP LOCKED state, so that device behaves differently
>>>>> from before. Then why put it in IOMMUFD?
>>>>
>>>>
>>>> "TSM bind" sets up the CPU side of it, it binds a VM to a piece of
>>>> IOMMU on the host CPU. The device does not know about the VM, it
>>>> just enables/disables encryption by a request from the CPU (those
>>>> start/stop interface commands). And IOMMUFD won't be doing DOE, the
>>>> platform driver (such as AMD CCP) will. Nothing to do for VFIO here.
>>>>
>>>> We probably should notify VFIO about the state transition but I do
>>>> not know VFIO would want to do in response.
>>>
>>> We have an awkward fit for what CCA people are doing to the various
>>> Linux APIs. Looking somewhat maximally across all the arches a "bind"
>>> for a CC vPCI device creation operation does:
>>>
>>>   - Setup the CPU page tables for the VM to have access to the MMIO
>>>   - Revoke hypervisor access to the MMIO
>>>   - Setup the vIOMMU to understand the vPCI device
>>>   - Take over control of some of the IOVA translation, at least for
>>> T=1, and route to the the vIOMMU
>>>   - Register the vPCI with any attestation functions the VM might use
>>>   - Do some DOE stuff to manage/validate TDSIP/etc
>>>
>>> So we have interactions of things controlled by PCI, KVM, VFIO, and
>>> iommufd all mushed together.
>>>
>>> iommufd is the only area that already has a handle to all the required
>>> objects:
>>>   - The physical PCI function
>>>   - The CC vIOMMU object
>>>   - The KVM FD
>>>   - The CC vPCI object
>>>
>>> Which is why I have been thinking it is the right place to manage
>>> this.
>>>
>>> It doesn't mean that iommufd is suddenly doing PCI stuff, no, that
>>> stays in VFIO.
>>>
>>>>>> So your issue is you need to shoot down the dmabuf during vPCI
>>>>>> device destruction?
>>>>>
>>>>> I assume "vPCI device" refers to assigned device in both shared
>>>>> mode & prvate mode. So no, I need to shoot down the dmabuf during
>>>>> TSM unbind, a.k.a. when assigned device is converting from
>>>>> private to shared. Then recover the dmabuf after TSM unbind. The
>>>>> device could still work in VM in shared mode.
>>>
>>> What are you trying to protect with this? Is there some intelism where
>>> you can't have references to encrypted MMIO pages?
>>>
>>
>> I think it is a matter of design choice. The encrypted MMIO page is
>> related to the TDI context and secure second level translation table
>> (S-EPT). and S-EPT is related to the confidential VM's context.
>>
>> AMD and ARM have another level of HW control, together
>> with a TSM-owned meta table, can simply mask out the access to those
>> encrypted MMIO pages. Thus, the life cycle of the encrypted mappings in
>> the second level translation table can be de-coupled from the TDI
>> unbound. They can be reaped un-harmfully later by hypervisor in another
>> path.
>>
>> While on Intel platform, it doesn't have that additional level of
>> HW control by design. Thus, the cleanup of encrypted MMIO page mapping
>> in the S-EPT has to be coupled tightly with TDI context destruction in
>> the TDI unbind process.
>>
>> If the TDI unbind is triggered in VFIO/IOMMUFD, there has be a
>> cross-module notification to KVM to do cleanup in the S-EPT.
> 
> QEMU should know about this unbind and can tell KVM about it too. No 
> cross module notification needed, it is not a hot path.
> 

Yes. QEMU knows almost everything important, it can do the required flow 
and kernel can enforce the requirements. There shouldn't be problem at 
runtime.

But if QEMU crashes, what are left there are only fd closing paths and 
objects that fds represent in the kernel. The modules those fds belongs 
need to solve the dependencies of tearing down objects without the help 
of QEMU.

There will be private MMIO dmabuf fds, VFIO fds, IOMMU device fd, KVM
fds at that time. Who should trigger the TDI unbind at this time?

I think it should be triggered in the vdevice teardown path in IOMMUfd
fd closing path, as it is where the bind is initiated.

iommufd vdevice tear down (iommu fd closing path)
     ----> tsm_tdi_unbind
         ----> intel_tsm_tdi_unbind
             ...
             ----> private MMIO un-maping in KVM
                 ----> cleanup private MMIO mapping in S-EPT and others
                 ----> signal MMIO dmabuf can be safely removed.
                        ^TVM teardown path (dmabuf uninstall path) checks
                        this state and wait before it can decrease the
                        dmabuf fd refcount
             ...
         ----> KVM TVM fd put
     ----> continue iommufd vdevice teardown.

Also, I think we need:

iommufd vdevice TSM bind
     ---> tsm_tdi_bind
         ----> intel_tsm_tdi_bind
             ...
             ----> KVM TVM fd get
             ...

Z.

> 
>> So shooting down the DMABUF object (encrypted MMIO page) means shooting
>> down the S-EPT mapping and recovering the DMABUF object means
>> re-construct the non-encrypted MMIO mapping in the EPT after the TDI is
>> unbound.
> 
> This is definitely QEMU's job to re-mmap MMIO to the userspace (as it 
> does for non-trusted devices today) so later on nested page fault could 
> fill the nested PTE. Thanks,
> 
> 
>>
>> Z.
>>
>>>>> What I really want is, one SW component to manage MMIO dmabuf,
>>>>> secure iommu & TSM bind/unbind. So easier coordinate these 3
>>>>> operations cause these ops are interconnected according to secure
>>>>> firmware's requirement.
>>>>
>>>> This SW component is QEMU. It knows about FLRs and other config
>>>> space things, it can destroy all these IOMMUFD objects and talk to
>>>> VFIO too, I've tried, so far it is looking easier to manage. Thanks,
>>>
>>> Yes, qemu should be sequencing this. The kernel only needs to enforce
>>> any rules required to keep the system from crashing.
>>>
>>> Jason
>>>
>>
>