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

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Date:   Thu, 5 Aug 2021 08:27:13 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.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

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.

Jason