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

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Date:   Wed, 29 Sep 2021 20:43:01 -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 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