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

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Date:   Tue, 1 Jun 2021 12:04:00 +0000
From:   Parav Pandit <parav@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Tian, Kevin" <kevin.tian@...el.com>,
        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: Monday, May 31, 2021 11:43 PM
> 
> On Mon, May 31, 2021 at 05:37:35PM +0000, Parav Pandit wrote:
> 
> > In that case, can it be a new system call? Why does it have to be under
> /dev/ioasid?
> > For example few years back such system call mpin() thought was proposed
> in [1].
> 
> Reference counting of the overall pins are required
> 
> So when a pinned pages is incorporated into an IOASID page table in a later
> IOCTL it means it cannot be unpinned while the IOASID page table is using it.
Ok. but cant it use the same refcount of that mmu uses?

> 
> This is some trick to organize the pinning into groups and then refcount each
> group, thus avoiding needing per-page refcounts.
Pinned page refcount is already maintained by the mmu without ioasid, isn't it?

> 
> The data structure would be an interval tree of pins in general
> 
> The ioasid itself would have an interval tree of its own mappings, each entry
> in this tree would reference count against an element in the above tree
> 
> Then the ioasid's interval tree would be mapped into a page table tree in HW
> format.
Does it mean in simple use case [1], second level page table copy is maintained in the IOMMU side via map interface?
I hope not. It should use the same as what mmu uses, right?

[1] one SIOV/ADI device assigned with one PASID and mapped in guest VM

> 
> The redundant storages are needed to keep track of the refencing and the
> CPU page table values for later unpinning.
> 
> Jason