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

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Date:   Wed, 29 Sep 2021 15:57:20 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.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>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

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_?

Or, it can be done by the platform IOMMU code where system PASID is
automatically enabled for PASID capable devices during boot and stored in
struct device. Device drivers can retrieve the PASID from struct device.

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).

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

> Jason


Thanks,

Jacob