linux-kernel - Re: [PATCH v2 5/8] iommu: Add PASID support for DMA mapping API users

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Date:   Tue, 15 Mar 2022 11:22:16 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        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>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        Jacob Pan <jacob.jun.pan@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 5/8] iommu: Add PASID support for DMA mapping API users

On Tue, Mar 15, 2022 at 11:16:41AM +0000, Robin Murphy wrote:
> On 2022-03-15 05:07, Jacob Pan wrote:
> > DMA mapping API is the de facto standard for in-kernel DMA. It operates
> > on a per device/RID basis which is not PASID-aware.
> > 
> > Some modern devices such as Intel Data Streaming Accelerator, PASID is
> > required for certain work submissions. To allow such devices use DMA
> > mapping API, we need the following functionalities:
> > 1. Provide device a way to retrieve a PASID for work submission within
> > the kernel
> > 2. Enable the kernel PASID on the IOMMU for the device
> > 3. Attach the kernel PASID to the device's default DMA domain, let it
> > be IOVA or physical address in case of pass-through.
> > 
> > This patch introduces a driver facing API that enables DMA API
> > PASID usage. Once enabled, device drivers can continue to use DMA APIs as
> > is. There is no difference in dma_handle between without PASID and with
> > PASID.
> 
> Surely the main point of PASIDs is to be able to use more than one
> of them?

IMHO, not for the DMA API.

I can't think of good reasons why a single in-kernel device should
require more than one iommu_domain for use by the DMA API. Even with
the SIOV cases we have been looking at we don't really see a use case
for more than one DMA API iommu_domain on a single physical device.
Do you know of something on the horizon?

>From my view the main point of PASIDs is to assign iommu_domains that
are not used by the DMA API.

IMHO it is a device mis-design of IDXD to require all DMA be PASID
tagged. Devices should be able to do DMA on their RID when the PCI
function is controlled by a kernel driver. I see this driver facing
API as addressing a device quirk by aliasing the DMA API of the RID
into a PASID and that is really all it is good for.

In any case I think we are better to wait for an actual user for multi
DMA API iommu_domains to come forward before we try to build an API
for it.

Jason