linux-kernel - Re: [PATCH v4 19/22] vfio-pci: Register an iommu fault handler

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Date:   Mon, 25 Feb 2019 18:30:45 +0100
From:   Auger Eric <eric.auger@...hat.com>
To:     Vincent Stehlé <vincent.stehle@....com>
Cc:     eric.auger.pro@...il.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, joro@...tes.org,
        alex.williamson@...hat.com, jacob.jun.pan@...ux.intel.com,
        yi.l.liu@...ux.intel.com, jean-philippe.brucker@....com,
        will.deacon@....com, robin.murphy@....com, kevin.tian@...el.com,
        ashok.raj@...el.com, marc.zyngier@....com,
        christoffer.dall@....com, peter.maydell@...aro.org
Subject: Re: [PATCH v4 19/22] vfio-pci: Register an iommu fault handler

Hi Vincent,

On 2/25/19 3:22 PM, Vincent Stehlé wrote:
> Hi Eric,
> 
> On Mon, Feb 18, 2019 at 02:55:00PM +0100, Eric Auger wrote:
>> This patch registers a fault handler which records faults in
>> a circular buffer and then signals an eventfd. This buffer is
>> exposed within the fault region.
>>
>> Signed-off-by: Eric Auger <eric.auger@...hat.com>
>> ---
>>  drivers/vfio/pci/vfio_pci.c         | 49 +++++++++++++++++++++++++++++
>>  drivers/vfio/pci/vfio_pci_private.h |  1 +
>>  2 files changed, 50 insertions(+)
>>
>> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
>> index aaf63e5ca2b6..019c9fd380a5 100644
>> --- a/drivers/vfio/pci/vfio_pci.c
>> +++ b/drivers/vfio/pci/vfio_pci.c
> (..)
>>  static int vfio_pci_init_fault_region(struct vfio_pci_device *vdev)
>>  {
>>  	struct vfio_region_fault_prod *header;
>> @@ -276,6 +317,13 @@ static int vfio_pci_init_fault_region(struct vfio_pci_device *vdev)
>>  	header = (struct vfio_region_fault_prod *)vdev->fault_pages;
>>  	header->version = -1;
>>  	header->offset = PAGE_SIZE;
>> +
>> +	ret = iommu_register_device_fault_handler(&vdev->pdev->dev,
>> +					vfio_pci_iommu_dev_fault_handler,
>> +					vdev);
>> +	if (ret)
>> +		goto out;
>> +
>>  	return 0;
>>  out:
>>  	kfree(vdev->fault_pages);
> 
> This patch calls iommu_register_device_fault_handler from
> vfio_pci_init_fault_region, leading to the following call stack:
> 
> iommu_register_device_fault_handler
> vfio_pci_init_fault_region
> vfio_pci_enable
> vfio_pci_open
> vfio_group_get_device_fd
> 
>> @@ -1420,6 +1468,7 @@ static void vfio_pci_remove(struct pci_dev *pdev)
>>  	vfio_iommu_group_put(pdev->dev.iommu_group, &pdev->dev);
>>  	kfree(vdev->region);
>>  	kfree(vdev->fault_pages);
>> +	iommu_unregister_device_fault_handler(&pdev->dev);
>>  	mutex_destroy(&vdev->ioeventfds_lock);
>>  	kfree(vdev);
> 
> And then this patch calls iommu_unregister_device_fault_handler from
> vfio_pci_remove, and not from vfio_pci_release.

Yes you're fully right. Thank you for the time spent debugging the
issue. this is a left-over from the previous version and indeed the
unregistration should be called from the release ops.

By the way, I will package a qemu version for testing this week. Sorry
for the delay.

Thanks

Eric
> 
> I think this means a device cannot be used twice in a row without unloading the
> module.
> 
> Here is an example sequence:
> 
> 1. modprobe vfio-pci
> 2. Userspace uses VFIO, calls ioctl(VFIO_GROUP_GET_DEVICE_FD)
>     2.1. iommu_register_device_fault_handler is called
> 3. Userspace exits
>     3.1. vfio_pci_release is called,
>          but iommu_unregister_device_fault_handler is not called
> 4. Userspace uses VFIO agin, calls ioctl(VFIO_GROUP_GET_DEVICE_FD) again
>     4.1. iommu_register_device_fault_handler is called again,
>          notices a fault handler is already there,
>          returns -EBUSY
> 
> Unloading the vfio-pci module will call vfio_pci_remove.
> 
> Maybe iommu_unregister_device_fault_handler should be called from
> vfio_pci_release instead of vfio_pci_remove?
> 
> Best regards,
> Vincent.
>