[PATCH v2] iommu: intel: do deep dma-unmapping, to avoid kernel-flooding.

[Ajay Garg <ajaygargnsit@gmail.com>]

Origins at :
https://lists.linuxfoundation.org/pipermail/iommu/2021-October/thread.html

=== Changes from v1 => v2 ===

a)
Improved patch-description.

b)
A more root-level fix, as suggested by

	1.
	Alex Williamson <alex.williamson@redhat.com>

	2.
	Lu Baolu <baolu.lu@linux.intel.com>



=== Issue ===

Kernel-flooding is seen, when an x86_64 L1 guest (Ubuntu-21) is booted in qemu/kvm
on a x86_64 host (Ubuntu-21), with a host-pci-device attached.

Following kind of logs, along with the stacktraces, cause the flood :

......
 DMAR: ERROR: DMA PTE for vPFN 0x428ec already set (to 3f6ec003 not 3f6ec003)
 DMAR: ERROR: DMA PTE for vPFN 0x428ed already set (to 3f6ed003 not 3f6ed003)
 DMAR: ERROR: DMA PTE for vPFN 0x428ee already set (to 3f6ee003 not 3f6ee003)
 DMAR: ERROR: DMA PTE for vPFN 0x428ef already set (to 3f6ef003 not 3f6ef003)
 DMAR: ERROR: DMA PTE for vPFN 0x428f0 already set (to 3f6f0003 not 3f6f0003)
......



=== Current Behaviour, leading to the issue ===

Currently, when we do a dma-unmapping, we unmap/unlink the mappings, but
the pte-entries are not cleared.

Thus, following sequencing would flood the kernel-logs :

i)
A dma-unmapping makes the real/leaf-level pte-slot invalid, but the 
pte-content itself is not cleared.

ii)
Now, during some later dma-mapping procedure, as the pte-slot is about
to hold a new pte-value, the intel-iommu checks if a prior 
pte-entry exists in the pte-slot. If it exists, it logs a kernel-error,
along with a corresponding stacktrace.

iii)
Step ii) runs in abundance, and the kernel-logs run insane.



=== Fix ===

We ensure that as part of a dma-unmapping, each (unmapped) pte-slot
is also cleared of its value/content (at the leaf-level, where the 
real mapping from a iova => pfn mapping is stored). 

This completes a "deep" dma-unmapping.



Signed-off-by: Ajay Garg <ajaygargnsit@gmail.com>
---
 drivers/iommu/intel/iommu.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index d75f59ae28e6..485a8ea71394 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -5090,6 +5090,8 @@ static size_t intel_iommu_unmap(struct iommu_domain *domain,
 	gather->freelist = domain_unmap(dmar_domain, start_pfn,
 					last_pfn, gather->freelist);
 
+	dma_pte_clear_range(dmar_domain, start_pfn, last_pfn);
+
 	if (dmar_domain->max_addr == iova + size)
 		dmar_domain->max_addr = iova;
 
-- 
2.30.2

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