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From: Lu Baolu <>
To: Jon Derrick <>,
	Joerg Roedel <>,
Cc:, Bjorn Helgaas <>,, Daniel Drake <>,
Subject: Re: [PATCH 1/1] iommu/vt-d: use DMA domain for real DMA devices and subdevices
Date: Fri, 10 Apr 2020 09:22:08 +0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>


On 2020/4/10 3:17, Jon Derrick wrote:
> The PCI devices handled by intel-iommu may have a DMA requester on another bus,
> such as VMD subdevices needing to use the VMD endpoint.
> The real DMA device is now used for the DMA mapping, but one case was missed
> earlier: if the VMD device (and hence subdevices too) are under
> IOMMU_DOMAIN_IDENTITY, mappings do not work.
> Codepaths like intel_map_page() handle the IOMMU_DOMAIN_DMA case by creating an
> iommu DMA mapping, and fall back on dma_direct_map_page() for the
> IOMMU_DOMAIN_IDENTITY case. However, handling of the IDENTITY case is broken
> when intel_page_page() handles a subdevice.
> We observe that at iommu attach time, dmar_insert_one_dev_info() for the
> subdevices will never set dev->archdata.iommu. This is because that function

Do you mind telling why not setting this?

> uses find_domain() to check if there is already an IOMMU for the device, and
> find_domain() then defers to the real DMA device which does have one. Thus
> dmar_insert_one_dev_info() returns without assigning dev->archdata.iommu.
> Then, later:
> 1. intel_map_page() checks if an IOMMU mapping is needed by calling
>     iommu_need_mapping() on the subdevice. identity_mapping() returns
>     false because dev->archdata.iommu is NULL, so this function
>     returns false indicating that mapping is needed.
> 2. __intel_map_single() is called to create the mapping.
> 3. __intel_map_single() calls find_domain(). This function now returns
>     the IDENTITY domain corresponding to the real DMA device.
> 4. __intel_map_single() calls domain_get_iommu() on this "real" domain.
>     A failure is hit and the entire operation is aborted, because this
>     codepath is not intended to handle IDENTITY mappings:
>         if (WARN_ON(domain->domain.type != IOMMU_DOMAIN_DMA))
>                     return NULL;

This is caused by the fragile private domain implementation. We are in
process of removing it by enhancing the iommu subsystem with per-group
default domain.

So ultimately VMD subdevices should have their own per-device iommu data
and support per-device dma ops.

Best regards,

  reply	other threads:[~2020-04-10  1:22 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-09 19:17 [PATCH 0/1] Real DMA dev DMA domain patch Jon Derrick
2020-04-09 19:17 ` [PATCH 1/1] iommu/vt-d: use DMA domain for real DMA devices and subdevices Jon Derrick
2020-04-10  1:22   ` Lu Baolu [this message]
2020-04-13  2:25     ` Daniel Drake
2020-04-13  2:48       ` Lu Baolu
2020-04-13 16:08         ` Derrick, Jonathan
2020-04-18 12:13         ` Joerg Roedel
2020-04-12  3:50   ` Daniel Drake
2020-04-12  7:35     ` Christoph Hellwig
2020-04-13 16:08     ` Derrick, Jonathan

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