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

Alexey Kardashevskiy
2024-10-01T01:56:13Z

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,

>
>>> stage-2 IO pagetable. Each VIOMMU then just need to only allocate its own
>>> VMID to attach the shared stage-2 IO pagetable to the physical IOMMU:
>>
>> this reads like 'VMID' is a virtual ID allocated by vIOMMU. But from the
>> entire context it actually means the physical 'VMID' allocated on the
>> associated physical IOMMU, correct?
>
> Quoting Jason's narratives, a VMID is a "Security namespace for
> guest owned ID". The allocation, using SMMU as an example, should
> be a part of vIOMMU instance allocation in the host SMMU driver.
> Then, this VMID will be used to mark the cache tags. So, it is
> still a software allocated ID, while HW would use it too.
>
> Thanks
> Nicolin

--
Alexey