linux-kernel - [PATCH] iommu: intel: remove flooding of non-error logs, when new-DMA-PTE is the same as old-DMA-PTE.

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Message-Id: <20211002124012.18186-1-ajaygargnsit@gmail.com>
Date:   Sat,  2 Oct 2021 18:10:12 +0530
From:   Ajay Garg <ajaygargnsit@...il.com>
To:     linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        kvm@...r.kernel.org
Cc:     Ajay Garg <ajaygargnsit@...il.com>
Subject: [PATCH] iommu: intel: remove flooding of non-error logs, when new-DMA-PTE is the same as old-DMA-PTE.

Taking a SD-MMC controller (over PCI) as an example, following is an
example sequencing, where the log-flooding happened :

0.
We have a host and a guest, both running latest x86_64 kernels.

1.
Host-machine is booted up (with intel_iommu=on), and the DMA-PTEs
are setup for the controller (on the host), for the first time.

2.
The SD-controller device is added to a (L1) guest on a KVM-VM
(via virt-manager).

3.
The KVM-VM is booted up.

4.
Above triggers a re-setup of DMA-PTEs on the host, for a
second time.

It is observed that the new PTEs formed (on the host) are same
as the original PTEs, and thus following logs, accompanied by
stacktraces, overwhelm the logs :

......
 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)
......

As the PTEs are same, so there is no cause of concern, and we can easily
avoid the logs-flood for this non-error case.

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

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index d75f59ae28e6..8bea8b4e3ff9 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -2370,7 +2370,7 @@ __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
 		 * touches the iova range
 		 */
 		tmp = cmpxchg64_local(&pte->val, 0ULL, pteval);
-		if (tmp) {
+		if (tmp && (tmp != pteval)) {
 			static int dumps = 5;
 			pr_crit("ERROR: DMA PTE for vPFN 0x%lx already set (to %llx not %llx)\n",
 				iov_pfn, tmp, (unsigned long long)pteval);
-- 
2.30.2