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Date:   Thu, 22 Jul 2021 13:30:38 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
Cc:     Shenming Lu <lushenming@huawei.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Jason Wang <jasowang@redhat.com>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
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        David Gibson <david@gibson.dropbear.id.au>,
        Robin Murphy <robin.murphy@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Woodhouse <dwmw2@infradead.org>
Subject: Re: [RFC v2] /dev/iommu uAPI proposal
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On Wed, Jul 21, 2021 at 02:13:23AM +0000, Tian, Kevin wrote:
> > From: Shenming Lu
> > Sent: Friday, July 16, 2021 8:20 PM
> > 
> > On 2021/7/16 9:20, Tian, Kevin wrote:
> >  > To summarize, for vIOMMU we can work with the spec owner to
> > > define a proper interface to feedback such restriction into the guest
> > > if necessary. For the kernel part, it's clear that IOMMU fd should
> > > disallow two devices attached to a single [RID] or [RID, PASID] slot
> > > in the first place.
> > >
> > > Then the next question is how to communicate such restriction
> > > to the userspace. It sounds like a group, but different in concept.
> > > An iommu group describes the minimal isolation boundary thus all
> > > devices in the group can be only assigned to a single user. But this
> > > case is opposite - the two mdevs (both support ENQCMD submission)
> > > with the same parent have problem when assigned to a single VM
> > > (in this case vPASID is vm-wide translated thus a same pPASID will be
> > > used cross both mdevs) while they instead work pretty well when
> > > assigned to different VMs (completely different vPASID spaces thus
> > > different pPASIDs).
> > >
> > > One thought is to have vfio device driver deal with it. In this proposal
> > > it is the vfio device driver to define the PASID virtualization policy and
> > > report it to userspace via VFIO_DEVICE_GET_INFO. The driver understands
> > > the restriction thus could just hide the vPASID capability when the user
> > > calls GET_INFO on the 2nd mdev in above scenario. In this way the
> > > user even doesn't need to know such restriction at all and both mdevs
> > > can be assigned to a single VM w/o any problem.
> > >
> > 
> > The restriction only probably happens when two mdevs are assigned to one
> > VM,
> > how could the vfio device driver get to know this info to accurately hide
> > the vPASID capability for the 2nd mdev when VFIO_DEVICE_GET_INFO?
> > There is no
> > need to do this in other cases.
> > 
> 
> I suppose the driver can detect it via whether two mdevs are opened by a
> single process.

Just have the kernel some ID for the PASID numberspace - devices with
the same ID have to be represented as a single RID.

Jason