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

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Date:   Thu, 3 Jun 2021 02:11:53 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        David Gibson <david@...son.dropbear.id.au>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        "Lu Baolu" <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>, Jason Wang <jasowang@...hat.com>,
        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>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>
Subject: RE: [RFC] /dev/ioasid uAPI proposal

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, June 3, 2021 12:17 AM
>
[...] 
> > > If there are no hypervisor traps (does this exist?) then there is no
> > > way to involve the hypervisor here and the child IOASID should simply
> > > be a pointer to the guest's data structure that describes binding. In
> > > this case that IOASID should claim all PASIDs when bound to a
> > > RID.
> >
> > And in that case I think we should call that object something other
> > than an IOASID, since it represents multiple address spaces.
> 
> Maybe.. It is certainly a special case.
> 
> We can still consider it a single "address space" from the IOMMU
> perspective. What has happened is that the address table is not just a
> 64 bit IOVA, but an extended ~80 bit IOVA formed by "PASID, IOVA".

More accurately 64+20=84 bit IOVA 😊

> 
> If we are already going in the direction of having the IOASID specify
> the page table format and other details, specifying that the page

I'm leaning toward this direction now, after a discussion with Baolu.
He reminded me that a default domain is already created for each
device when it's probed by the iommu driver. So it looks workable
to expose a per-device capability query uAPI to user once a device
is bound to the ioasid fd. Once it's available, the user should be able
to judge what format/mode should be set when creating an IOASID.

> tabnle format is the 80 bit "PASID, IOVA" format is a fairly small
> step.

In concept this view is true. But when designing the uAPI possibly
we will not call it a 84bit format as the PASID table itself just
serves 20bit PASID space. 

Will think more how to mark it in the next version.

> 
> I wouldn't twist things into knots to create a difference, but if it
> is easy to do it wouldn't hurt either.
> 

Thanks
Kevin