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

[Alexey Kardashevskiy]

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@xxxxxxxxxx>
> > > > > 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