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

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Date:   Thu, 5 Aug 2021 22:44:19 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     David Gibson <david@...son.dropbear.id.au>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>,
        "Jean-Philippe Brucker" <jean-philippe@...aro.org>,
        Jason Wang <jasowang@...hat.com>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shenming Lu <lushenming@...wei.com>,
        Joerg Roedel <joro@...tes.org>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "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>
Subject: RE: [RFC v2] /dev/iommu uAPI proposal

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, August 5, 2021 7:27 PM
> 
> On Wed, Aug 04, 2021 at 10:59:21PM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Wednesday, August 4, 2021 10:05 PM
> > >
> > > On Mon, Aug 02, 2021 at 02:49:44AM +0000, Tian, Kevin wrote:
> > >
> > > > Can you elaborate? IMO the user only cares about the label (device
> cookie
> > > > plus optional vPASID) which is generated by itself when doing the
> attaching
> > > > call, and expects this virtual label being used in various spots
> (invalidation,
> > > > page fault, etc.). How the system labels the traffic (the physical RID or
> RID+
> > > > PASID) should be completely invisible to userspace.
> > >
> > > I don't think that is true if the vIOMMU driver is also emulating
> > > PASID. Presumably the same is true for other PASID-like schemes.
> > >
> >
> > I'm getting even more confused with this comment. Isn't it the
> > consensus from day one that physical PASID should not be exposed
> > to userspace as doing so breaks live migration?
> 
> Uh, no?
> 
> > with PASID emulation vIOMMU only cares about vPASID instead of
> > pPASID, and the uAPI only requires user to register vPASID instead
> > of reporting pPASID back to userspace...
> 
> vPASID is only a feature of one device in existance, so we can't make
> vPASID mandatory.
> 

sure. my point is just that if vPASID is being emulated there is no need
of exposing pPASID to user space. Can you give a concrete example
where pPASID must be exposed and how the user wants to use this
information? 

Thanks
Kevin