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Date:   Thu, 23 Sep 2021 10:01:35 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
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Subject: Re: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID_ALLOC/FREE
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On Thu, Sep 23, 2021 at 12:45:17PM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Thursday, September 23, 2021 8:31 PM
> > 
> > On Thu, Sep 23, 2021 at 12:22:23PM +0000, Tian, Kevin wrote:
> > > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > > Sent: Thursday, September 23, 2021 8:07 PM
> > > >
> > > > On Thu, Sep 23, 2021 at 09:14:58AM +0000, Tian, Kevin wrote:
> > > >
> > > > > currently the type is aimed to differentiate three usages:
> > > > >
> > > > > - kernel-managed I/O page table
> > > > > - user-managed I/O page table
> > > > > - shared I/O page table (e.g. with mm, or ept)
> > > >
> > > > Creating a shared ios is something that should probably be a different
> > > > command.
> > >
> > > why? I didn't understand the criteria here...
> > 
> > I suspect the input args will be very different, no?
> 
> yes, but can't the structure be extended to incorporate it? 

You need to be thoughtful, giant structures with endless combinations
of optional fields turn out very hard. I haven't even seen what args
this shared thing will need, but I'm guessing it is almost none, so
maybe a new call is OK?

If it is literally just 'give me an ioas for current mm' then it has
no args or complexity at all.

> > > > > we can remove 'type', but is FORMAT_KENREL/USER/SHARED a good
> > > > > indicator? their difference is not about format.
> > > >
> > > > Format should be
> > > >
> > > > FORMAT_KERNEL/FORMAT_INTEL_PTE_V1/FORMAT_INTEL_PTE_V2/etc
> > >
> > > INTEL_PTE_V1/V2 are formats. Why is kernel-managed called a format?
> > 
> > So long as we are using structs we need to have values then the field
> > isn't being used. FORMAT_KERNEL is a reasonable value to have when we
> > are not creating a userspace page table.
> > 
> > Alternatively a userspace page table could have a different API
> 
> I don't know. Your comments really confused me on what's the right
> way to design the uAPI. If you still remember, the original v1 proposal
> introduced different uAPIs for kernel/user-managed cases. Then you
> recommended to consolidate everything related to ioas in one allocation
> command.

This is because you had almost completely duplicated the input args
between the two calls.

If it turns out they have very different args, then they should have
different calls.

> > > - open iommufd
> > > - create an ioas
> > > - attach vfio device to ioasid, with vPASID info
> > > 	* vfio converts vPASID to pPASID and then call
> > iommufd_device_attach_ioasid()
> > > 	* the latter then installs ioas to the IOMMU with RID/PASID
> > 
> > This was your flow for mdev's, I've always been talking about wanting
> > to see this supported for all use cases, including physical PCI
> > devices w/ PASID support.
> 
> this is not a flow for mdev. It's also required for pdev on Intel platform,
> because the pasid table is in HPA space thus must be managed by host 
> kernel. Even no translation we still need the user to provide the pasid info.

There should be no mandatory vPASID stuff in most of these flows, that
is just a special thing ENQCMD virtualization needs. If userspace
isn't doing ENQCMD virtualization it shouldn't need to touch this
stuff.

> as explained earlier, on Intel platform the user always needs to provide 
> a PASID in the attaching call. whether it's directly used (for pdev)
> or translated (for mdev) is the underlying driver thing. From kernel
> p.o.v, since this PASID is provided by the user, it's fine to call it vPASID
> in the uAPI.

I've always disagreed with this. There should be an option for the
kernel to pick an appropriate PASID for portability to other IOMMUs
and simplicity of the interface.

You need to keep it clear what is in the minimum basic path and what
is needed for special cases, like ENQCMD virtualization.

Not every user of iommufd is doing virtualization.

Jason