RE: [RFC v2] /dev/iommu uAPI proposal

From: "Tian, Kevin" <kevin.tian@intel.com>
To: Shenming Lu <lushenming@huawei.com>
Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>,
	"Jiang, Dave" <dave.jiang@intel.com>,
	"Raj, Ashok" <ashok.raj@intel.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Jonathan Corbet <corbet@lwn.net>,
	David Woodhouse <dwmw2@infradead.org>,
	Jason Wang <jasowang@redhat.com>,
	"Alex Williamson \(alex.williamson@redhat.com\)"
	<alex.williamson@redhat.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Kirti Wankhede <kwankhede@nvidia.com>,
	"Enrico Weigelt, metux IT consult" <lkml@metux.net>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	"wanghaibin.wang@huawei.com" <wanghaibin.wang@huawei.com>,
	David Gibson <david@gibson.dropbear.id.au>,
	Robin Murphy <robin.murphy@arm.com>,
	"parav@mellanox.com" <parav@mellanox.com>
Subject: RE: [RFC v2] /dev/iommu uAPI proposal
Date: Wed, 21 Jul 2021 02:13:23 +0000	[thread overview]
Message-ID: <BN9PR11MB54332CA3CF19835A7B2742688CE39@BN9PR11MB5433.namprd11.prod.outlook.com> (raw)
In-Reply-To: <013e240d-f627-3565-aba1-71b2d6f514b4@huawei.com>

> 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.

Thanks
Kevin
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