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

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Date:   Tue, 1 Jun 2021 08:10:14 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
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>,
        David Gibson <david@...son.dropbear.id.au>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "Robin Murphy" <robin.murphy@....com>
Subject: RE: [RFC] /dev/ioasid uAPI proposal

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Saturday, May 29, 2021 1:36 AM
> 
> On Thu, May 27, 2021 at 07:58:12AM +0000, Tian, Kevin wrote:
> 
> > IOASID nesting can be implemented in two ways: hardware nesting and
> > software nesting. With hardware support the child and parent I/O page
> > tables are walked consecutively by the IOMMU to form a nested translation.
> > When it's implemented in software, the ioasid driver is responsible for
> > merging the two-level mappings into a single-level shadow I/O page table.
> > Software nesting requires both child/parent page tables operated through
> > the dma mapping protocol, so any change in either level can be captured
> > by the kernel to update the corresponding shadow mapping.
> 
> Why? A SW emulation could do this synchronization during invalidation
> processing if invalidation contained an IOVA range.

In this proposal we differentiate between host-managed and user-
managed I/O page tables. If host-managed, the user is expected to use
map/unmap cmd explicitly upon any change required on the page table. 
If user-managed, the user first binds its page table to the IOMMU and 
then use invalidation cmd to flush iotlb when necessary (e.g. typically
not required when changing a PTE from non-present to present).

We expect user to use map+unmap and bind+invalidate respectively
instead of mixing them together. Following this policy, map+unmap
must be used in both levels for software nesting, so changes in either 
level are captured timely to synchronize the shadow mapping.

> 
> I think this document would be stronger to include some "Rational"
> statements in key places
> 

Sure. I tried to provide rationale as much as possible but sometimes 
it's lost in a complex context like this. :)

Thanks
Kevin