linux-kernel - Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

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Date:   Wed, 29 Sep 2021 16:39:53 -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>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        mike.campin@...el.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

On Wed, Sep 29, 2021 at 12:37:19PM -0700, Jacob Pan wrote:

> For #2, it seems we can store the kernel PASID in struct device. This will
> preserve the DMA API interface while making it PASID capable. Essentially,
> each PASID capable device would have two special global PASIDs: 
> 	- PASID 0 for DMA request w/o PASID, aka RID2PASID
> 	- PASID 1 (randomly selected) for in-kernel DMA request w/ PASID

This seems reasonable, I had the same thought. Basically just have the
driver issue some trivial call:
  pci_enable_pasid_dma(pdev, &pasid)

And then DMA tagged with the PASID will be handled equivilant to
untagged DMA. Basically PASID and no PASID point to the exact same IO
page table and the DMA API manipulates that single page table.

Having multiple RID's pointing at the same IO page table is something
we expect iommufd to require so the whole thing should ideally fall
out naturally.

Jason