linux-kernel - Re: [PATCH 2/4] iommu: Add PASID support for DMA mapping API users

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Date:   Wed, 8 Dec 2021 10:49:39 -0800
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        "Christoph Hellwig" <hch@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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>, Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Barry Song <21cnbao@...il.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH 2/4] iommu: Add PASID support for DMA mapping API users

Hi Lu,

On Wed, 8 Dec 2021 10:31:36 +0800, Lu Baolu <baolu.lu@...ux.intel.com>
wrote:

> Hi Jacob,
> 
> On 12/7/21 9:47 PM, 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 kernel PASID for work submission
> > 2. Enable the kernel PASID on the IOMMU
> > 3. Establish address space for the kernel PASID that matches the default
> >     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.  
> 
> Can a device issue DMA requests with PASID even there's no system IOMMU
> or the system IOMMU is disabled?
> 
Good point. 
If IOMMU is not enabled, device cannot issue DMA requests with PASID. This
API will not be available. Forgot to add dummy functions to the header.

> Best regards,
> baolu


Thanks,

Jacob