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

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

On 2023/8/22 16:24, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@linux.intel.com>
>> Sent: Tuesday, August 22, 2023 2:37 PM
>>
>> 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.
>>
> 
> If that's the case better add a comment to include this fact. So
> another one looking at this code won't fall into the same puzzle
> wondering what about a group becoming non-singleton after
> above check. 😊

Yeah, fair enough.

Best regards,
baolu