linux-kernel - Re: Plan for /dev/ioasid RFC v2

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Message-ID: <20210617230438.GZ1002214@nvidia.com>
Date:   Thu, 17 Jun 2021 20:04:38 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Gibson <david@...son.dropbear.id.au>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        "Tian, Kevin" <kevin.tian@...el.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>,
        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: Plan for /dev/ioasid RFC v2

On Thu, Jun 17, 2021 at 03:02:33PM +1000, David Gibson wrote:

> In other words, do we really have use cases where we need to identify
> different devices IDs, even though we know they're not isolated.

I think when PASID is added in and all the complexity that brings, it
does become more important, yes.

At the minimum we should scope the complexity.

I'm not convinced it is so complicated, really it is just a single bit
of information toward userspace: 'all devices in this group must use
the same IOASID'

Something like qemu consumes this bit and creates the pci/pcie bridge
to model this to the guest and so on.

Something like dpdk just doesn't care (same as today).

Jason