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

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Date:   Thu, 30 Sep 2021 14:22:34 +0000
From:   "Campin, Mike" <mike.campin@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC:     "iommu@...ts.linux-foundation.org" <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>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: RE: [RFC 0/7] Support in-kernel DMA with PASID and SVA

I need support for mixed user PASID, kernel PASID and non-PASID use cases in the driver.

-----Original Message-----
From: Jason Gunthorpe <jgg@...dia.com> 
Sent: Wednesday, September 29, 2021 4:43 PM
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>; Luck, Tony <tony.luck@...el.com>; Jiang, Dave <dave.jiang@...el.com>; Raj, Ashok <ashok.raj@...el.com>; Kumar, Sanjay K <sanjay.k.kumar@...el.com>; Campin, Mike <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 03:57:20PM -0700, Jacob Pan wrote:
> Hi Jason,
> 
> On Wed, 29 Sep 2021 16:39:53 -0300, Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> > 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)
> That would work, but I guess it needs to be an iommu_ call instead of pci_?

Which ever makes sense..  The API should take in a struct pci_device and return a PCI PASID - at least as a wrapper around a more generic immu api.

> I think your suggestion is more precise, in case the driver does not 
> want to do DMA w/ PASID, we can do less IOTLB flush (PASID 0 only).

Since it is odd, and it may create overhead, I would do it only when asked to do it

> > 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.

> That would be the equivalent of attaching multiple devices to the same 
> IOMMU domain. right?

Effectively..

Jason