linux-kernel - RE: [RFC v2] /dev/iommu uAPI proposal

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Message-ID: <BN9PR11MB5433D6313D75EA10919ABE818CE39@BN9PR11MB5433.namprd11.prod.outlook.com>
Date:   Wed, 21 Jul 2021 02:11:12 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Raj, Ashok" <ashok.raj@...el.com>,
        Shenming Lu <lushenming@...wei.com>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Jason Wang <jasowang@...hat.com>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        "wanghaibin.wang@...wei.com" <wanghaibin.wang@...wei.com>
Subject: RE: [RFC v2] /dev/iommu uAPI proposal

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Saturday, July 17, 2021 2:30 AM
> 
> On Fri, Jul 16, 2021 at 01:20:15AM +0000, Tian, Kevin wrote:
> 
> > 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.
> 
> I think it makes more sense to expose some kind of "pasid group" to
> qemu that identifies that each PASID must be unique across the
> group. For vIOMMUs that are doing funky things with the RID This means
> a single PASID group must not be exposed as two RIDs to the guest.
> 

It's an interesting idea. Some aspects are still unclear to me now 
e.g. how to describe such restriction in a way that it's applied only
to a single user owning the group (not the case when assigned to
different users), whether it can be generalized cross subsystems
(vPASID being a vfio-managed resource), etc. Let's refine it when 
working on the actual implementation. 

> If the kernel blocks it then it can never be fixed by updating the
> vIOMMU design.
> 

But the mdev driver can choose to do so. Should we prevent it?

btw just be curious whether you have got a chance to have a full 
review on v2. I wonder when might be a good time to discuss 
the execution plan following this proposal, if no major open remains...

Thanks
Kevin