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

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Date:   Wed, 16 Mar 2022 19:15:50 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     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>,
        Jacob Pan <jacob.jun.pan@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, Yi Liu <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 01:50:04PM -0700, Jacob Pan wrote:

> I guess a list of (device, pasid) tuples as you suggested could work but it
> will have duplicated device entries since each device could have multiple
> PASIDs. right?

Is assigning the same iommu_domain to multiple PASIDs of the same
device something worth optimizing for?

I would expect real applications will try to use the same PASID for
the same IOVA map to optimize IOTLB caching.

Is there a use case for that I'm missing?

Otherwise your explanation is what I was imagining as well.

I would also think about expanding your struct so that the device
driver can track per-device per-domain data as well, that seems
useful IIRC?

ie put a 'sizeof_iommu_dev_pasid_data' in the domain->ops and
allocate that much memory so the driver can use the trailer space for
its own purpose.

Jason