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 09:13:58 -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 Tue, Dec 07, 2021 at 05:47:14AM -0800, Jacob Pan wrote:
> In-kernel DMA should be managed by DMA mapping API. The existing kernel
> PASID support is based on the SVA machinery in SVA lib that is intended
> for user process SVA. The binding between a kernel PASID and kernel
> mapping has many flaws. See discussions in the link below.
> 
> 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..

> +	/*
> +	 * Try to enable both in-kernel and user DMA request with PASID.
> +	 * PASID is supported unless both user and kernel PASID are
> +	 * supported. Do not fail probe here in that idxd can still be
> +	 * used w/o PASID or IOMMU.
> +	 */
> +	if (iommu_dev_enable_feature(dev, IOMMU_DEV_FEAT_SVA) ||
> +		idxd_enable_system_pasid(idxd)) {
> +		dev_warn(dev, "Failed to enable PASID\n");
> +	} else {
> +		set_bit(IDXD_FLAG_PASID_ENABLED, &idxd->flags);
>  	}

Huh? How can the driver keep going if PASID isn't supported? I thought
the whole point of this was because the device cannot do DMA without
PASID at all?

Jason