linux-kernel - Re: [PATCH 2/3] iommu/vt-d: Apply per-device dma

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Date:   Tue, 6 Aug 2019 08:43:47 +0200
From:   Christoph Hellwig <hch@....de>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>, ashok.raj@...el.com,
        jacob.jun.pan@...el.com, kevin.tian@...el.com,
        Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
Subject: Re: [PATCH 2/3] iommu/vt-d: Apply per-device dma_ops

Hi Lu,

I really do like the switch to the per-device dma_map_ops, but:

On Thu, Aug 01, 2019 at 02:01:55PM +0800, Lu Baolu wrote:
> Current Intel IOMMU driver sets the system level dma_ops. This
> implementation has at least the following drawbacks: 1) each
> dma API will go through the IOMMU driver even the devices are
> using identity mapped domains; 2) if user requests to use an
> identity mapped domain (a.k.a. bypass iommu translation), the
> driver might fall back to dma domain blindly if the device is
> not able to address all system memory.

This is very clearly a behavioral regression.  The intel-iommu driver
has always used the iommu mapping to provide decent support for
devices that do not have the full 64-bit addressing capability, and
changing this will make a lot of existing setups go slower.

I don't think having to use swiotlb for these devices helps anyone.