linux-kernel - Re: [PATCH v2 3/8] iommu/vt-d: Implement device

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Date:   Wed, 16 Mar 2022 21:04:10 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>
Subject: Re: [PATCH v2 3/8] iommu/vt-d: Implement device_pasid domain attach
 ops

On Wed, Mar 16, 2022 at 10:23:26PM +0000, Luck, Tony wrote:

> Kernel users (ring0) can supply any PASID when they use
> the ENQCMDS instruction. Is that what you mean when you
> say "real applications"?

I'm not talking about ENQCMD at all

I'm saying I don't see much utility to have two PASIDs assigned to the
same device that perform the same IOVA translation.

So I would expect the list of attachments in an iommu_domain to have
exactly one device in it and multiple PASIDs to the same device from
the same iommu_domain is a oddball corner case we don't need to
optimize for, beyond being able to allow it to happen.

Jason