[PATCH v4] iommu/vt-d: consider real PCI device when checking if mapping is needed

[Daniel Drake <drake@endlessm.com>]

From: Jon Derrick <jonathan.derrick@intel.com>

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 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;

Fix this by using the real DMA device when checking if a mapping is
needed. The IDENTITY case will then directly fall back on
dma_direct_map_page(). The subdevice DMA mask is still considered in
order to handle any situations where (e.g.) the subdevice only supports
32-bit DMA with the real DMA requester having a 64-bit DMA mask.

Reported-by: Daniel Drake <drake@endlessm.com>
Fixes: b0140c69637e ("iommu/vt-d: Use pci_real_dma_dev() for mapping")
Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Daniel Drake <drake@endlessm.com>
---

Notes:
    v2: switch to Jon's approach instead.
    v3: shortcut mask check in non-identity case
    v4: amend commit msg to explain why subdevice DMA mask is still considered
    
    This problem was originally detected with a non-upstream patch which
    creates PCI devices similar to VMD:
    "PCI: Add Intel remapped NVMe device support"
    (https://marc.info/?l=linux-ide&m=156015271021615&w=2)
    
    This patch has now been tested on VMD forced into identity mode.

 drivers/iommu/intel-iommu.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index 9dc37672bf89..7ffd252bf835 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -3582,12 +3582,16 @@ static struct dmar_domain *get_private_domain_for_dev(struct device *dev)
 /* Check if the dev needs to go through non-identity map and unmap process.*/
 static bool iommu_need_mapping(struct device *dev)
 {
+	struct device *dma_dev = dev;
 	int ret;
 
 	if (iommu_dummy(dev))
 		return false;
 
-	ret = identity_mapping(dev);
+	if (dev_is_pci(dev))
+		dma_dev = &pci_real_dma_dev(to_pci_dev(dev))->dev;
+
+	ret = identity_mapping(dma_dev);
 	if (ret) {
 		u64 dma_mask = *dev->dma_mask;
 
@@ -3601,19 +3605,19 @@ static bool iommu_need_mapping(struct device *dev)
 		 * 32 bit DMA is removed from si_domain and fall back to
 		 * non-identity mapping.
 		 */
-		dmar_remove_one_dev_info(dev);
-		ret = iommu_request_dma_domain_for_dev(dev);
+		dmar_remove_one_dev_info(dma_dev);
+		ret = iommu_request_dma_domain_for_dev(dma_dev);
 		if (ret) {
 			struct iommu_domain *domain;
 			struct dmar_domain *dmar_domain;
 
-			domain = iommu_get_domain_for_dev(dev);
+			domain = iommu_get_domain_for_dev(dma_dev);
 			if (domain) {
 				dmar_domain = to_dmar_domain(domain);
 				dmar_domain->flags |= DOMAIN_FLAG_LOSE_CHILDREN;
 			}
-			dmar_remove_one_dev_info(dev);
-			get_private_domain_for_dev(dev);
+			dmar_remove_one_dev_info(dma_dev);
+			get_private_domain_for_dev(dma_dev);
 		}
 
 		dev_info(dev, "32bit DMA uses non-identity mapping\n");
-- 
2.20.1

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