linux-kernel - Re: [PATCH v4 1/6] iommu: Add a per domain PASID for DMA API

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Message-ID: <20220531160309.GG1343366@nvidia.com>
Date:   Tue, 31 May 2022 13:03:09 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Christoph Hellwig <hch@...radead.org>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "will@...nel.org" <will@...nel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Eric Auger <eric.auger@...hat.com>
Subject: Re: [PATCH v4 1/6] iommu: Add a per domain PASID for DMA API

On Tue, May 31, 2022 at 08:45:28PM +0800, Baolu Lu wrote:

> My understanding is that device driver owns its PASID policy.

I'm not sure this is actually useful, the core code should provide the
allocator as I can't think of any device that actually needs a special
allocator.

> If ENQCMD is supported on the device, the PASIDs should be allocated
> through ioasid_alloc(). Otherwise, the whole PASID pool is managed
> by the device driver.

This is OK only if we know in-advance that the device needs a global
PASID. ENQCMD is one reason, maybe there are others down the road.

Better to make this core code policy.

Jason