linux-kernel - Re: [RFC 06/20] iommu: Add iommu_device_init[exit]_user

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Date:   Wed, 29 Sep 2021 10:38:36 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: Re: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma
 interfaces

On 9/29/21 10:29 AM, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Wednesday, September 29, 2021 10:22 AM
>>
>> On 9/28/21 10:07 PM, Jason Gunthorpe wrote:
>>> On Tue, Sep 28, 2021 at 09:35:05PM +0800, Lu Baolu wrote:
>>>> Another issue is, when putting a device into user-dma mode, all devices
>>>> belonging to the same iommu group shouldn't be bound with a kernel-
>> dma
>>>> driver. Kevin's prototype checks this by READ_ONCE(dev->driver). This is
>>>> not lock safe as discussed below,
>>>>
>>>> https://lore.kernel.org/linux-
>> iommu/20210927130935.GZ964074@...dia.com/
>>>>
>>>> Any guidance on this?
>>>
>>> Something like this?
>>>
>>>
>>> int iommu_set_device_dma_owner(struct device *dev, enum
>> device_dma_owner mode,
>>> 			       struct file *user_owner)
>>> {
>>> 	struct iommu_group *group = group_from_dev(dev);
>>>
>>> 	spin_lock(&iommu_group->dma_owner_lock);
>>> 	switch (mode) {
>>> 		case DMA_OWNER_KERNEL:
>>> 			if (iommu_group-
>>> dma_users[DMA_OWNER_USERSPACE])
>>> 				return -EBUSY;
>>> 			break;
>>> 		case DMA_OWNER_SHARED:
>>> 			break;
>>> 		case DMA_OWNER_USERSPACE:
>>> 			if (iommu_group-
>>> dma_users[DMA_OWNER_KERNEL])
>>> 				return -EBUSY;
>>> 			if (iommu_group->dma_owner_file != user_owner) {
>>> 				if (iommu_group-
>>> dma_users[DMA_OWNER_USERSPACE])
>>> 					return -EPERM;
>>> 				get_file(user_owner);
>>> 				iommu_group->dma_owner_file =
>> user_owner;
>>> 			}
>>> 			break;
>>> 		default:
>>> 			spin_unlock(&iommu_group->dma_owner_lock);
>>> 			return -EINVAL;
>>> 	}
>>> 	iommu_group->dma_users[mode]++;
>>> 	spin_unlock(&iommu_group->dma_owner_lock);
>>> 	return 0;
>>> }
>>>
>>> int iommu_release_device_dma_owner(struct device *dev,
>>> 				   enum device_dma_owner mode)
>>> {
>>> 	struct iommu_group *group = group_from_dev(dev);
>>>
>>> 	spin_lock(&iommu_group->dma_owner_lock);
>>> 	if (WARN_ON(!iommu_group->dma_users[mode]))
>>> 		goto err_unlock;
>>> 	if (!iommu_group->dma_users[mode]--) {
>>> 		if (mode == DMA_OWNER_USERSPACE) {
>>> 			fput(iommu_group->dma_owner_file);
>>> 			iommu_group->dma_owner_file = NULL;
>>> 		}
>>> 	}
>>> err_unlock:
>>> 	spin_unlock(&iommu_group->dma_owner_lock);
>>> }
>>>
>>>
>>> Where, the driver core does before probe:
>>>
>>>      iommu_set_device_dma_owner(dev, DMA_OWNER_KERNEL, NULL)
>>>
>>> pci_stub/etc does in their probe func:
>>>
>>>      iommu_set_device_dma_owner(dev, DMA_OWNER_SHARED, NULL)
>>>
>>> And vfio/iommfd does when a struct vfio_device FD is attached:
>>>
>>>      iommu_set_device_dma_owner(dev, DMA_OWNER_USERSPACE,
>> group_file/iommu_file)
>>
>> Really good design. It also helps alleviating some pains elsewhere in
>> the iommu core.
>>
>> Just a nit comment, we also need DMA_OWNER_NONE which will be set
>> when
>> the driver core unbinds the driver from the device.
>>
> 
> Not necessarily. NONE is represented by none of dma_user[mode]
> is valid.
> 

Fair enough.

Best regards,
baolu