Re: [PATCH v2 01/12] iommu/vt-d: debugfs: Remove device_domain_lock usage

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2022/6/14 14:43, Tian, Kevin wrote:
>> From: Lu Baolu<baolu.lu@linux.intel.com>
>> Sent: Tuesday, June 14, 2022 10:51 AM
>>
>> The domain_translation_struct debugfs node is used to dump the DMAR
>> page
>> tables for the PCI devices. It potentially races with setting domains to
>> devices. The existing code uses a global spinlock device_domain_lock to
>> avoid the races, but this is problematical as this lock is only used to
>> protect the device tracking lists of each domain.
> is it really problematic at this point? Before following patches are applied
> using device_domain_lock should have similar effect as holding the group
> lock.
> 
> Here it might make more sense to just focus on removing the use of
> device_domain_lock outside of iommu.c. Just that using group lock is
> cleaner and more compatible to following cleanups.
> 
> and it's worth mentioning that racing with page table updates is out
> of the scope of this series. Probably also add a comment in the code
> to clarify this point.
> 

Hi Kevin,

How do you like below updated patch?

 From cecc9a0623780a11c4ea4d0a15aa6187f01541c4 Mon Sep 17 00:00:00 2001
From: Lu Baolu <baolu.lu@linux.intel.com>
Date: Sun, 29 May 2022 10:18:56 +0800
Subject: [PATCH 1/1] iommu/vt-d: debugfs: Remove device_domain_lock usage

The domain_translation_struct debugfs node is used to dump the DMAR page
tables for the PCI devices. It potentially races with setting domains to
devices. The existing code uses the global spinlock device_domain_lock to
avoid the races.

This removes the use of device_domain_lock outside of iommu.c by replacing
it with the group mutex lock. Using the group mutex lock is cleaner and
more compatible to following cleanups.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
---
  drivers/iommu/intel/debugfs.c | 42 +++++++++++++++++++++++++----------
  drivers/iommu/intel/iommu.c   |  2 +-
  drivers/iommu/intel/iommu.h   |  1 -
  3 files changed, 31 insertions(+), 14 deletions(-)

diff --git a/drivers/iommu/intel/debugfs.c b/drivers/iommu/intel/debugfs.c
index d927ef10641b..f4acd8993f60 100644
--- a/drivers/iommu/intel/debugfs.c
+++ b/drivers/iommu/intel/debugfs.c
@@ -342,13 +342,13 @@ static void pgtable_walk_level(struct seq_file *m, 
struct dma_pte *pde,
  	}
  }

-static int show_device_domain_translation(struct device *dev, void *data)
+static int __show_device_domain_translation(struct device *dev, void *data)
  {
-	struct device_domain_info *info = dev_iommu_priv_get(dev);
-	struct dmar_domain *domain = info->domain;
+	struct dmar_domain *domain;
  	struct seq_file *m = data;
  	u64 path[6] = { 0 };

+	domain = to_dmar_domain(iommu_get_domain_for_dev(dev));
  	if (!domain)
  		return 0;

@@ -359,20 +359,38 @@ static int show_device_domain_translation(struct 
device *dev, void *data)
  	pgtable_walk_level(m, domain->pgd, domain->agaw + 2, 0, path);
  	seq_putc(m, '\n');

-	return 0;
+	return 1;
  }

-static int domain_translation_struct_show(struct seq_file *m, void *unused)
+static int show_device_domain_translation(struct device *dev, void *data)
  {
-	unsigned long flags;
-	int ret;
+	struct iommu_group *group;

-	spin_lock_irqsave(&device_domain_lock, flags);
-	ret = bus_for_each_dev(&pci_bus_type, NULL, m,
-			       show_device_domain_translation);
-	spin_unlock_irqrestore(&device_domain_lock, flags);
+	group = iommu_group_get(dev);
+	if (group) {
+		/*
+		 * The group->mutex is held across the callback, which will
+		 * block calls to iommu_attach/detach_group/device. Hence,
+		 * the domain of the device will not change during traversal.
+		 *
+		 * All devices in an iommu group share a single domain, hence
+		 * we only dump the domain of the first device. Even though,
+		 * this code still possibly races with the iommu_unmap()
+		 * interface. This could be solved by RCU-freeing the page
+		 * table pages in the iommu_unmap() path.
+		 */
+		iommu_group_for_each_dev(group, data,
+					 __show_device_domain_translation);
+		iommu_group_put(group);
+	}

-	return ret;
+	return 0;
+}
+
+static int domain_translation_struct_show(struct seq_file *m, void *unused)
+{
+	return bus_for_each_dev(&pci_bus_type, NULL, m,
+				show_device_domain_translation);
  }
  DEFINE_SHOW_ATTRIBUTE(domain_translation_struct);

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 19024dc52735..a39d72a9d1cf 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -314,7 +314,7 @@ static int iommu_skip_te_disable;
  #define IDENTMAP_GFX		2
  #define IDENTMAP_AZALIA		4

-DEFINE_SPINLOCK(device_domain_lock);
+static DEFINE_SPINLOCK(device_domain_lock);
  static LIST_HEAD(device_domain_list);

  /*
diff --git a/drivers/iommu/intel/iommu.h b/drivers/iommu/intel/iommu.h
index a22adfbdf870..8a6d64d726c0 100644
--- a/drivers/iommu/intel/iommu.h
+++ b/drivers/iommu/intel/iommu.h
@@ -480,7 +480,6 @@ enum {
  #define VTD_FLAG_SVM_CAPABLE		(1 << 2)

  extern int intel_iommu_sm;
-extern spinlock_t device_domain_lock;

  #define sm_supported(iommu)	(intel_iommu_sm && ecap_smts((iommu)->ecap))
  #define pasid_supported(iommu)	(sm_supported(iommu) &&			\
-- 
2.25.1

Best regards,
baolu