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 17:12:23 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        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>,
        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 12/8/2021 4:39 PM, Jason Gunthorpe wrote:
> 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?

DSA is different than typical DMA devices in the past. Instead of a 
software descriptor ring where the device DMA to fetch the descriptors 
after the software ringing a doorbell or writing a head index, the 
descriptors are submitted directly to the device via a CPU instruction 
(i.e. MOVDIR64B or ENQCMD(S)). The CPU takes the KVA of the 64B 
descriptor and writes to the device atomically. No DMA mapping is 
necessary in this case.


>
>>> 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