Re: [PATCH v2 1/3] iommu: Make single-device group for PASID explicit

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

On 2023/8/21 14:33, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@linux.intel.com>
>> Sent: Monday, August 21, 2023 1:45 PM
>>
>> On 2023/8/18 11:56, Tian, Kevin wrote:
>>>> From: Lu Baolu <baolu.lu@linux.intel.com>
>>>> Sent: Monday, August 14, 2023 9:18 AM
>>>>
>>>> The PASID interfaces have always supported only single-device groups.
>>>> This was first introduced in commit 26b25a2b98e45 ("iommu: Bind
>> process
>>>> address spaces to devices"), and has been kept consistent in subsequent
>>>> commits.
>>>>
>>>> However, the core code doesn't explicitly check for this requirement
>>>> after commit 201007ef707a8 ("PCI: Enable PASID only when ACS RR & UF
>>>> enabled on upstream path"), which made this requirement implicit.
>>>>
>>>> Restore the check to make it explicit that the PASID interfaces only
>>>> support devices belonging to single-device groups.
>>>>
>>>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>>>> ---
>>>>    drivers/iommu/iommu.c | 5 +++++
>>>>    1 file changed, 5 insertions(+)
>>>>
>>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>>>> index 71b9c41f2a9e..f1eba60e573f 100644
>>>> --- a/drivers/iommu/iommu.c
>>>> +++ b/drivers/iommu/iommu.c
>>>> @@ -3408,6 +3408,11 @@ int iommu_attach_device_pasid(struct
>>>> iommu_domain *domain,
>>>>    		return -ENODEV;
>>>>
>>>>    	mutex_lock(&group->mutex);
>>>> +	if (list_count_nodes(&group->devices) != 1) {
>>>> +		ret = -EINVAL;
>>>> +		goto out_unlock;
>>>> +	}
>>>> +
>>>
>>> I wonder whether we should also block adding new device to this
>>> group once the single-device has pasid enabled. Otherwise the
>>
>> This has been guaranteed by pci_enable_pasid():
>>
>>           if (!pci_acs_path_enabled(pdev, NULL, PCI_ACS_RR | PCI_ACS_UF))
>>                   return -EINVAL;
>>
> 
> well since you are adding generic core check then it's not good to
> rely on the fact of a specific bus...

We attempted to do this in the patch linked below.

https://lore.kernel.org/linux-iommu/20220705050710.2887204-5-baolu.lu@linux.intel.com/

After long discussion, we decided to move it to the pci_enable_pasid()
interface. The non-static single device group is only relevant to PCI
fabrics that support hot-plugging without ACS support on the upstream
path.

Best regards,
baolu