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

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

On 2022/6/15 14:13, Tian, Kevin wrote:
>> From: Baolu Lu<baolu.lu@linux.intel.com>
>> Sent: Wednesday, June 15, 2022 9:54 AM
>>
>> 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?
> Yes, this is better.
> 
>>   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,
> bus_for_each_dev() will still lead to duplicated dump in the same group
> but probably we can leave with it for a debug interface.
> 

Yes. This is what it was. Ideally we could walk the iommu groups and
dump the device names belonging to the group and it's domain mappings,
but I was not willing to add any helpers in the iommu core just for a
debugfs use.

---
Best regards,
baolu