linux-kernel - Re: [PATCH v2 3/3] iommufd/device: Change iommufd_hw_pagetable_has

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Date:   Wed, 1 Feb 2023 14:49:08 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Nicolin Chen <nicolinc@...dia.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "Lu, Baolu" <baolu.lu@...el.com>,
        "robin.murphy@....com" <robin.murphy@....com>
Cc:     baolu.lu@...ux.intel.com, "Liu, Yi L" <yi.l.liu@...el.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] iommufd/device: Change
 iommufd_hw_pagetable_has_group to device centric

On 2023/2/1 11:07, Tian, Kevin wrote:
>> From: Nicolin Chen <nicolinc@...dia.com>
>> Sent: Monday, January 30, 2023 6:22 PM
>>
>> On Sun, Jan 29, 2023 at 02:38:55AM -0800, Nicolin Chen wrote:
>>
>>>>> @@ -385,10 +372,8 @@ void iommufd_device_detach(struct
>>>>> iommufd_device *idev)
>>>>>        struct iommufd_hw_pagetable *hwpt = idev->hwpt;
>>>>>
>>>>>        mutex_lock(&hwpt->ioas->mutex);
>>>>> -     mutex_lock(&hwpt->devices_lock);
>>>>>        refcount_dec(hwpt->devices_users);
>>>>> -     list_del(&idev->devices_item);
>>>>> -     if (!iommufd_hw_pagetable_has_group(hwpt, idev->group)) {
>>>>> +     if (iommufd_hw_pagetable_has_device(hwpt, idev->dev)) {
>>>>>                if (refcount_read(hwpt->devices_users) == 1) {
>>>>>                        iopt_table_remove_domain(&hwpt->ioas->iopt,
>>>>>                                                 hwpt->domain);
>>>>> @@ -397,7 +382,6 @@ void iommufd_device_detach(struct
>> iommufd_device
>>>>> *idev)
>>>>>                iommu_detach_group(hwpt->domain, idev->group);
>>>>>        }
>>>>
>>>> emmm how do we track last device detach in a group? Here the first
>>>> device detach already leads to group detach...
>>>
>>> Oh no. That's a bug. Thanks for catching it.
>>>
>>> We need an additional refcount somewhere to track the number of
>>> attached devices in the iommu_group.
>>
>> Wondering if we can let iommu_attach/detach_device handle this:
>>
> 
> that is the desired way to fully remove group awareness in iommufd.
> 
> but iirc there were some concerns on changing their semantics. But
> I don't remember the detail now. Jason might know. also +Baolu/Robin.
> 
> otherwise as long as the group attach/detach continues to be used
> then identifying last device in the group always needs some hack
> within iommufd itself.

I have tried to solve this problem.

https://lore.kernel.org/linux-iommu/20220106022053.2406748-1-baolu.lu@linux.intel.com/

I may need to review the original discussion to see if I can update a
new version.

Best regards,
baolu