Re: [PATCH v2 5/8] iommu/vt-d: Make device pasid attachment explicit

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

On 3/28/23 3:44 PM, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@linux.intel.com>
>> Sent: Tuesday, March 28, 2023 1:49 PM
>>
>> On 3/28/23 7:21 AM, Jacob Pan wrote:
>>> diff --git a/drivers/iommu/intel/iommu.h b/drivers/iommu/intel/iommu.h
>>> index 65b15be72878..b6c26f25d1ba 100644
>>> --- a/drivers/iommu/intel/iommu.h
>>> +++ b/drivers/iommu/intel/iommu.h
>>> @@ -595,6 +595,7 @@ struct dmar_domain {
>>>
>>>    	spinlock_t lock;		/* Protect device tracking lists */
>>>    	struct list_head devices;	/* all devices' list */
>>> +	struct list_head dev_pasids;	/* all attached pasids */
>>>
>>>    	struct dma_pte	*pgd;		/* virtual address */
>>>    	int		gaw;		/* max guest address width */
>>> @@ -708,6 +709,7 @@ struct device_domain_info {
>>>    	u8 ats_supported:1;
>>>    	u8 ats_enabled:1;
>>>    	u8 dtlb_extra_inval:1;	/* Quirk for devices need extra flush */
>>> +	u8 dev_attached:1;	/* Device context activated */
>>>    	u8 ats_qdep;
>>>    	struct device *dev; /* it's NULL for PCIe-to-PCI bridge */
>>>    	struct intel_iommu *iommu; /* IOMMU used by this device */
>>> @@ -715,6 +717,12 @@ struct device_domain_info {
>>>    	struct pasid_table *pasid_table; /* pasid table */
>>>    };
>>>
>>> +struct device_pasid_info {
>>> +	struct list_head link_domain;	/* link to domain siblings */
>>> +	struct device *dev;		/* physical device derived from */
>>> +	ioasid_t pasid;			/* PASID on physical device */
>>> +};
>>
>> The dev_pasids list seems to be duplicate with iommu_group::pasid_array.
>>
>> The pasid_array is de facto per-device as the PCI subsystem requires ACS
>> to be enabled on the upstream path to the root port.
>>
>> pci_enable_pasid():
>> 385         if (!pci_acs_path_enabled(pdev, NULL, PCI_ACS_RR | PCI_ACS_UF))
>> 386                 return -EINVAL;
>>
>> For such PCI topology, pci_device_group() always assigns an exclusive
>> iommu group (a.k.a. singleton group).
>>
>> So, how about moving the pasid_array from struct iommu_group to struct
>> dev_iommu? With this refactoring, the individual iommu driver has no
>> need to create their own pasid array or list.
>>
>> Instead of using iommu_group::mutex, perhaps the pasid_array needs its
>> own lock in struct dev_iommu after moving.
>>
> 
> What you suggested is a right thing and more friendly to pasid attach
> in iommufd [1].
> 
> but dev_pasids list here is a different thing. It tracks which [device, pasid]
> is attached to the domain. w/o this information you'll have to walk the
> pasid_array of every attached device under the domain and search for
> every pasid entry pointing to the said domain. It's very inefficient.
> 
> of course if this can be done more generally it'd be nice.😊
> 
> [1] https://lore.kernel.org/linux-iommu/ZAjbDxSzxYPqSCjo@nvidia.com/

Ah, yes. You are right. I was confused.

Best regards,
baolu