linux-kernel - Re: [PATCH v3 1/3] iommu/vt-d: Allow 32bit devices to uses DMA domain

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Date:   Fri, 17 Apr 2020 08:50:54 +0200
From:   Christoph Hellwig <hch@....de>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Christoph Hellwig <hch@....de>, Joerg Roedel <joro@...tes.org>,
        ashok.raj@...el.com, jacob.jun.pan@...ux.intel.com,
        kevin.tian@...el.com,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Daniel Drake <drake@...lessm.com>,
        Derrick Jonathan <jonathan.derrick@...el.com>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v3 1/3] iommu/vt-d: Allow 32bit devices to uses DMA
 domain

On Thu, Apr 16, 2020 at 03:40:38PM +0800, Lu Baolu wrote:
>> description.  I'd need to look at the final code, but it seems like
>> this will still cause bounce buffering instead of using dynamic
>> mapping, which still seems like an awful idea.
>
> Yes. If the user chooses to use identity domain by default through
> kernel command, identity domain will be applied for all devices. For
> those devices with limited addressing capability, bounce buffering will
> be used when they try to access the memory beyond their address
> capability. This won't cause any kernel regression as far as I can see.
>
> Switching domain during runtime with drivers loaded will cause real
> problems as I said in the commit message. That's the reason why I am
> proposing to remove it. If we want to keep it, we have to make sure that
> switching domain for one device should not impact other devices which
> share the same domain with it. Furthermore, it's better to implement it
> in the generic layer to keep device driver behavior consistent on all
> architectures.

I don't disagree with the technical points.  What I pointed out is that

 a) the actual technical change is not in the commit log, which it
    should be
 b) that I still think taking away the ability to dynamically map
    devices in the identify domain after all the time we allowed for
    that is going to cause nasty regressions.