RE: [PATCH RFC v2 03/11] iommu/sva: Add iommu_domain type for SVA

From: "Tian, Kevin" <kevin.tian@intel.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: "Raj, Ashok" <ashok.raj@intel.com>, Will Deacon <will@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"Pan, Jacob jun" <jacob.jun.pan@intel.com>,
	Robin Murphy <robin.murphy@arm.com>
Subject: RE: [PATCH RFC v2 03/11] iommu/sva: Add iommu_domain type for SVA
Date: Wed, 6 Apr 2022 01:00:13 +0000	[thread overview]
Message-ID: <BN9PR11MB527660D99D154F922B0A628B8CE79@BN9PR11MB5276.namprd11.prod.outlook.com> (raw)
In-Reply-To: <20220402233210.GM2120790@nvidia.com>

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Sunday, April 3, 2022 7:32 AM
> 
> On Sat, Apr 02, 2022 at 08:43:16AM +0000, Tian, Kevin wrote:
> 
> > > This assumes any domain is interchangeable with any device, which is
> > > not the iommu model. We need a domain op to check if a device is
> > > compatiable with the domain for vfio an iommufd, this should do the
> > > same.
> >
> > This suggests that mm_struct needs to include the format information
> > of the CPU page table so the format can be checked by the domain op?
> 
> No, Linux does not support multiple formats for CPU page tables,
> AFAICT, and creating the SVA domain in the first place should check
> this.

One interesting usage is when virtio-iommu supports vSVA one day. At
that time there needs a way to know the format of the CPU page table
and then virtio-iommu driver needs to check whether it is compatible
with what the host iommu driver supports. But possibly this can wait to
be solved until that usage comes...

> 
> > > It means each mm can have a list of domains associated with it and a
> > > new domain is auto-created if the device doesn't work with any of the
> > > existing domains.
> >
> > mm has only one page table and one format. If a device is incompatible
> > with an existing domain wrapping that page table, how come creating
> > another domain could make it compatible?
> 
> Because domains wrap more than just the IOPTE format, they have
> additional data related to the IOMMU HW block itself. Imagine a SOC
> with two IOMMU HW blocks that can both process the CPU IOPTE format,
> but have different configuration.

Curious. Is it hypothesis or real? If real can you help give a concrete
example?

> 
> So if device A users IOMMU A it needs an iommu_domain from driver A and
> same for another device B, even if both iommu_domains are thin
> wrappers around the same mm_struct.
> 
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