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From: Christoph Hellwig <hch@lst.de>
To: Lu Baolu <baolu.lu@linux.intel.com>
Cc: kevin.tian@intel.com, ashok.raj@intel.com,
	linux-kernel@vger.kernel.org, Daniel Drake <drake@endlessm.com>,
	iommu@lists.linux-foundation.org,
	Robin Murphy <robin.murphy@arm.com>,
	Christoph Hellwig <hch@lst.de>,
	Derrick Jonathan <jonathan.derrick@intel.com>
Subject: Re: [PATCH v3 1/3] iommu/vt-d: Allow 32bit devices to uses DMA domain
Date: Fri, 17 Apr 2020 08:50:54 +0200	[thread overview]
Message-ID: <20200417065054.GA18880@lst.de> (raw)
In-Reply-To: <e11d8138-f704-2f5e-c0b1-70b367a33d5d@linux.intel.com>

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.

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WARNING: multiple messages have this Message-ID (diff)
From: Christoph Hellwig <hch@lst.de>
To: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Christoph Hellwig <hch@lst.de>, Joerg Roedel <joro@8bytes.org>,
	ashok.raj@intel.com, jacob.jun.pan@linux.intel.com,
	kevin.tian@intel.com,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
	iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
	Daniel Drake <drake@endlessm.com>,
	Derrick Jonathan <jonathan.derrick@intel.com>,
	Jerry Snitselaar <jsnitsel@redhat.com>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v3 1/3] iommu/vt-d: Allow 32bit devices to uses DMA domain
Date: Fri, 17 Apr 2020 08:50:54 +0200	[thread overview]
Message-ID: <20200417065054.GA18880@lst.de> (raw)
In-Reply-To: <e11d8138-f704-2f5e-c0b1-70b367a33d5d@linux.intel.com>

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.


  reply	other threads:[~2020-04-17  6:51 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-16  6:23 [PATCH v3 0/3] Replace private domain with per-group default domain Lu Baolu
2020-04-16  6:23 ` Lu Baolu
2020-04-16  6:23 ` [PATCH v3 1/3] iommu/vt-d: Allow 32bit devices to uses DMA domain Lu Baolu
2020-04-16  6:23   ` Lu Baolu
2020-04-16  7:01   ` Christoph Hellwig
2020-04-16  7:01     ` Christoph Hellwig
2020-04-16  7:40     ` Lu Baolu
2020-04-16  7:40       ` Lu Baolu
2020-04-17  6:50       ` Christoph Hellwig [this message]
2020-04-17  6:50         ` Christoph Hellwig
2020-04-17 12:49         ` Lu Baolu
2020-04-17 12:49           ` Lu Baolu
2020-04-16  6:23 ` [PATCH v3 2/3] iommu/vt-d: Allow PCI sub-hierarchy to use " Lu Baolu
2020-04-16  6:23   ` Lu Baolu
2020-04-16  6:23 ` [PATCH v3 3/3] iommu/vt-d: Apply per-device dma_ops Lu Baolu
2020-04-16  6:23   ` Lu Baolu

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