RE: [PATCH 3/6] vfio: remove the unused mdev iommu hook

From: "Tian, Kevin" <kevin.tian@intel.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Christoph Hellwig <hch@lst.de>, Joerg Roedel <joro@8bytes.org>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Lu Baolu <baolu.lu@linux.intel.com>,
	Will Deacon <will@kernel.org>,
	Kirti Wankhede <kwankhede@nvidia.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: RE: [PATCH 3/6] vfio: remove the unused mdev iommu hook
Date: Fri, 14 May 2021 12:58:10 +0000	[thread overview]
Message-ID: <MWHPR11MB18866205125E566FE05867A78C509@MWHPR11MB1886.namprd11.prod.outlook.com> (raw)
In-Reply-To: <20210514121925.GI1096940@ziepe.ca>

> From: Jason Gunthorpe <jgg@ziepe.ca>
> Sent: Friday, May 14, 2021 8:19 PM
> 
> On Fri, May 14, 2021 at 06:54:16AM +0000, Tian, Kevin wrote:
> > > From: Tian, Kevin
> > > Sent: Friday, May 14, 2021 2:28 PM
> > >
> > > > From: Jason Gunthorpe <jgg@ziepe.ca>
> > > > Sent: Thursday, May 13, 2021 8:01 PM
> > > >
> > > > On Thu, May 13, 2021 at 03:28:52AM +0000, Tian, Kevin wrote:
> > > >
> > > > > Are you specially concerned about this iommu_device hack which
> > > > > directly connects mdev_device to iommu layer or the entire removed
> > > > > logic including the aux domain concept? For the former we are now
> > > > > following up the referred thread to find a clean way. But for the latter
> > > > > we feel it's still necessary regardless of how iommu interface is
> > > redesigned
> > > > > to support device connection from the upper level driver. The reason
> is
> > > > > that with mdev or subdevice one physical device could be attached to
> > > > > multiple domains now. there could be a primary domain with
> DOMAIN_
> > > > > DMA type for DMA_API use by parent driver itself, and multiple
> auxiliary
> > > > > domains with DOMAIN_UNMANAGED types for subdevices assigned
> to
> > > > > different VMs.
> > > >
> > > > Why do we need more domains than just the physical domain for the
> > > > parent? How does auxdomain appear in /dev/ioasid?
> > > >
> > >
> > > Say the parent device has three WQs. WQ1 is used by parent driver itself,
> > > while WQ2/WQ3 are assigned to VM1/VM2 respectively.
> > >
> > > WQ1 is attached to domain1 for an IOVA space to support DMA API
> > > operations in parent driver.
> 
> More specifically WQ1 uses a PASID that is represented by an IOASID to
> userspace.

No. WQ1 is used by parent driver itself so it's not related to userspace. 

> 
> > > WQ2 is attached to domain2 for the GPA space of VM1. Domain2 is
> > > created when WQ2 is assigned to VM1 as a mdev.
> > >
> > > WQ3 is attached to domain3 for the GPA space of VM2. Domain3 is
> > > created when WQ3 is assigned to VM2 as a mdev.
> > >
> > > In this case domain1 is the primary while the other two are auxiliary
> > > to the parent.
> > >
> > > auxdomain represents as a normal domain in /dev/ioasid, with only
> > > care required when doing attachment.
> > >
> > > e.g. VM1 is assigned with both a pdev and mdev. Qemu creates
> > > gpa_ioasid which is associated with a single domain for VM1's
> > > GPA space and this domain is shared by both pdev and mdev.
> >
> > Here pdev/mdev are just conceptual description. Following your
> > earlier suggestion /dev/ioasid will not refer to explicit mdev_device.
> > Instead, each vfio device attached to an ioasid is represented by either
> > "struct device" for pdev or "struct device + pasid" for mdev. The
> > presence of pasid decides which iommu_attach api should be used.
> 
> But you still haven't explained what an aux domain is to /dev/ioasid.

'aux' vs. 'primary' matters only in the iommu layer. For /dev/ioasid
it is just normal iommu domain. The only attention required is to tell
the iommu layer whether this domain should be treated as 'aux' or
'primary' when attaching the domain to a device (based on pdev vs.
mdev). After this point, the domain is managed through existing 
iommu ops (map, unmap, etc.) just like how it works today, applying 
to all pdevs/mdevs attached to this ioasid

> 
> Why do I need more public kernel objects to represent a PASID IOASID?

This avoids changing every iommu ops to include a PASID and forcing
the upper-layer drivers to do it differently between pdev and mdev.
Actually this was a main motivation when working out aux domain 
proposal with Joerg two years ago.

> 
> Are you creating a domain for every IOASID? Why?
> 

Create a domain for every non-nesting ioasid. For nesting ioasid 
(e.g. ioasid1 on ioasid2), ioasid1 should inherit the domain from 
ioasid2.

The reason is that iommu domain represents an IOVA address
space shareable by multiple devices. It should be created at the 
point where the address space is managed. 

Thanks
Kevin

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