linux-kernel - Re: [PATCH 4/4] dmaengine: idxd: Use DMA API for in-kernel DMA with PASID

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Date:   Wed, 8 Dec 2021 19:39:36 -0400
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>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Jacob Pan <jacob.jun.pan@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.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>
Subject: Re: [PATCH 4/4] dmaengine: idxd: Use DMA API for in-kernel DMA with
 PASID

On Wed, Dec 08, 2021 at 01:59:45PM -0800, Jacob Pan wrote:
> Hi Jason,
> 
> On Wed, 8 Dec 2021 16:30:22 -0400, Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> > On Wed, Dec 08, 2021 at 11:55:16AM -0800, Jacob Pan wrote:
> > > Hi Jason,
> > > 
> > > On Wed, 8 Dec 2021 09:13:58 -0400, Jason Gunthorpe <jgg@...dia.com>
> > > wrote: 
> > > > > This patch utilizes iommu_enable_pasid_dma() to enable DSA to
> > > > > perform DMA requests with PASID under the same mapping managed by
> > > > > DMA mapping API. In addition, SVA-related bits for kernel DMA are
> > > > > removed. As a result, DSA users shall use DMA mapping API to obtain
> > > > > DMA handles instead of using kernel virtual addresses.    
> > > > 
> > > > Er, shouldn't this be adding dma_map/etc type calls?
> > > > 
> > > > You can't really say a driver is using the DMA API without actually
> > > > calling the DMA API..  
> > > The IDXD driver is not aware of addressing mode, it is up to the user of
> > > dmaengine API to prepare the buffer mappings. Here we only set up the
> > > PASID such that it can be picked up during DMA work submission. I
> > > tested with /drivers/dma/dmatest.c which does dma_map_page(),
> > > map_single etc. also tested with other pieces under development.  
> > 
> > Ignoring the work, doesn't IDXD prepare the DMA queues itself, don't
> > those need the DMA API?
> > 
> Do you mean wq completion record address? It is already using DMA API.
> 	wq->compls = dma_alloc_coherent(dev, wq->compls_size,
> &wq->compls_addr, GFP_KERNEL);
> 	desc->compl_dma = wq->compls_addr + idxd->data->compl_size * i;

I would have expected something on the queue submission side too?

> > the same thing, they do not use the same IOVA's. Did you test this
> > with bypass mode off?
> Yes with dmatest. IOVA is the default, I separated out the SATC patch which
> will put internal accelerators in bypass mode. It can also be verified by
> iommu debugfs dump of DMA PASID (2) and PASID 0 (RIDPASID) are pointing to
> he same default domain. e.g

Well, OK then..

Jason