Re: [PATCH v2 00/19] iommufd: Add VIOMMU infrastructure (Part-1)

Alexey Kardashevskiy
2024-10-01T05:07:13Z

On 1/10/24 13:36, Nicolin Chen wrote:
> On Tue, Oct 01, 2024 at 11:55:59AM +1000, Alexey Kardashevskiy wrote:
>> On 11/9/24 17:08, Nicolin Chen wrote:
>>> On Wed, Sep 11, 2024 at 06:12:21AM +0000, Tian, Kevin wrote:
>>>>> From: Nicolin Chen <nicolinc@nvidia.com>
>>>>> Sent: Wednesday, August 28, 2024 1:00 AM
>>>>>
>>>> [...]
>>>>> On a multi-IOMMU system, the VIOMMU object can be instanced to the
>>>>> number
>>>>> of vIOMMUs in a guest VM, while holding the same parent HWPT to share
>>>>> the
>>>>
>>>> Is there restriction that multiple vIOMMU objects can be only created
>>>> on a multi-IOMMU system?
>>>
>>> I think it should be generally restricted to the number of pIOMMUs,
>>> although likely (not 100% sure) we could do multiple vIOMMUs on a
>>> single-pIOMMU system. Any reason for doing that?
>>
>>
>> Just to clarify the terminology here - what are pIOMMU and vIOMMU exactly?
>>
>> On AMD, IOMMU is a pretend-pcie device, one per a rootport, manages a DT
>> - device table, one entry per BDFn, the entry owns a queue. A slice of
>> that can be passed to a VM (== queues mapped directly to the VM, and
>> such IOMMU appears in the VM as a pretend-pcie device too). So what is
>> [pv]IOMMU here? Thanks,
>
> The "p" stands for physical: the entire IOMMU unit/instance. In
> the IOMMU subsystem terminology, it's a struct iommu_device. It
> sounds like AMD would register one iommu device per rootport?

Yup, my test machine has 4 of these.

> The "v" stands for virtual: a slice of the pIOMMU that could be
> shared or passed through to a VM:
> - Intel IOMMU doesn't have passthrough queues, so it uses a
> shared queue (for invalidation). In this case, vIOMMU will
> be a pure SW structure for HW queue sharing (with the host
> machine and other VMs). That said, I think the channel (or
> the port) that Intel VT-d uses internally for a device to
> do a two-stage translation can be seen as a "passthrough"
> feature, held by a vIOMMU.
> - AMD IOMMU can assign passthrough queues to VMs, in which
> case, vIOMMU will be a structure holding all passthrough
> resource (of the pIOMMU) assisgned to a VM. If there is a
> shared resource, it can be packed into the vIOMMU struct
> too. FYI, vQUEUE (future series) on the other hand will
> represent each passthrough queue in a vIOMMU struct. The
> VM then, per that specific pIOMMU (rootport?), will have
> one vIOMMU holding a number of vQUEUEs.
> - ARM SMMU is sort of in the middle, depending on the impls.
> vIOMMU will be a structure holding both passthrough and
> shared resource. It can define vQUEUEs, if the impl has
> passthrough queues like AMD does.
>
> Allowing a vIOMMU to hold shared resource makes it a bit of an
> upgraded model for IOMMU virtualization, from the existing HWPT
> model that now looks like a subset of the vIOMMU model.

Thanks for confirming.

I've just read in this thread that "it should be generally restricted to
the number of pIOMMUs, although likely (not 100% sure) we could do
multiple vIOMMUs on a single-pIOMMU system. Any reason for doing that?"?
thought "we have every reason to do that, unless p means something
different", so I decided to ask :) Thanks,

>
> Thanks
> Nicolin

--
Alexey