Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma_ops

From: Lu Baolu <baolu.lu@linux.intel.com>
To: Christoph Hellwig <hch@lst.de>
Cc: alan.cox@intel.com, Stefano Stabellini <sstabellini@kernel.org>,
	ashok.raj@intel.com, Jonathan Corbet <corbet@lwn.net>,
	pengfei.xu@intel.com, Ingo Molnar <mingo@redhat.com>,
	David Woodhouse <dwmw2@infradead.org>,
	kevin.tian@intel.com,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	mika.westerberg@linux.intel.com, Juergen Gross <jgross@suse.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
	jacob.jun.pan@intel.com, Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma_ops
Date: Fri, 26 Jul 2019 09:56:51 +0800	[thread overview]
Message-ID: <a098359a-0f89-6028-68df-9f83718df256@linux.intel.com> (raw)
In-Reply-To: <20190725114348.GA30957@lst.de>

Hi,

On 7/25/19 7:43 PM, Christoph Hellwig wrote:
> On Thu, Jul 25, 2019 at 03:18:03PM +0800, Lu Baolu wrote:
>>> Don't we need to keep this bit so that we still allow the IOMMU
>>> to act if the device has a too small DMA mask to address all memory in
>>> the system, even if if it should otherwise be identity mapped?
>>>
>>
>> This checking happens only when device is using an identity mapped
>> domain. If the device has a small DMA mask, swiotlb will be used for
>> high memory access.
>>
>> This is supposed to be handled in dma_direct_map_page():
>>
>>          if (unlikely(!dma_direct_possible(dev, dma_addr, size)) &&
>>              !swiotlb_map(dev, &phys, &dma_addr, size, dir, attrs)) {
>>                  report_addr(dev, dma_addr, size);
>>                  return DMA_MAPPING_ERROR;
>>          }
> 
> Well, yes.  But the point is that the current code uses dynamic iommu
> mappings even if the devices is in the identity mapped domain when the
> dma mask іs too small to map all memory directly.  Your change means it
> will now use swiotlb which is most likely going to be a lot more

By default, we use DMA domain. The privileged users are able to change
this with global kernel parameter or per-group default domain type under
discussion. In another word, use of identity domain is a choice of the
privileged user who should consider the possible bounce buffer overhead.

I think current code doesn't do the right thing. The user asks the iommu
driver to use identity domain for a device, but the driver force it back
to DMA domain because of the device address capability.

> expensive.  I don't think that this change is a good idea, and even if
> we decide that this is a good idea after all that should be done in a
> separate prep patch that explains the rationale.

Yes. Make sense.

Best regards,
Baolu
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