Re: [RFC v3 18/21] vfio-pci: Add a new VFIO_REGION_TYPE_NESTED region type

[Auger Eric]

Hi Alex,

On 1/12/19 12:58 AM, Alex Williamson wrote:
> On Tue,  8 Jan 2019 11:26:30 +0100
> Eric Auger <eric.auger@xxxxxxxxxx> wrote:
> 
>> This patch adds a new 64kB region aiming to report nested mode
>> translation faults.
>>
>> The region contains a header with the size of the queue,
>> the producer and consumer indices and then the actual
>> fault queue data. The producer is updated by the kernel while
>> the consumer is updated by the userspace.
>>
>> Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx>
>>
>> ---
>> ---
>>  drivers/vfio/pci/vfio_pci.c         | 102 +++++++++++++++++++++++++++-
>>  drivers/vfio/pci/vfio_pci_private.h |   2 +
>>  include/uapi/linux/vfio.h           |  15 ++++
>>  3 files changed, 118 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
>> index ff60bd1ea587..2ba181ab2edd 100644
>> --- a/drivers/vfio/pci/vfio_pci.c
>> +++ b/drivers/vfio/pci/vfio_pci.c
>> @@ -56,6 +56,11 @@ module_param(disable_idle_d3, bool, S_IRUGO | S_IWUSR);
>>  MODULE_PARM_DESC(disable_idle_d3,
>>  		 "Disable using the PCI D3 low power state for idle, unused devices");
>>  
>> +#define VFIO_FAULT_REGION_SIZE 0x10000
> 
> Why 64K?
For the region to be mmappable with 64kB page size.
> 
>> +#define VFIO_FAULT_QUEUE_SIZE	\
>> +	((VFIO_FAULT_REGION_SIZE - sizeof(struct vfio_fault_region_header)) / \
>> +	sizeof(struct iommu_fault))
>> +
>>  static inline bool vfio_vga_disabled(void)
>>  {
>>  #ifdef CONFIG_VFIO_PCI_VGA
>> @@ -1226,6 +1231,100 @@ static const struct vfio_device_ops vfio_pci_ops = {
>>  static int vfio_pci_reflck_attach(struct vfio_pci_device *vdev);
>>  static void vfio_pci_reflck_put(struct vfio_pci_reflck *reflck);
>>  
>> +static size_t
>> +vfio_pci_dma_fault_rw(struct vfio_pci_device *vdev, char __user *buf,
>> +		      size_t count, loff_t *ppos, bool iswrite)
>> +{
>> +	unsigned int i = VFIO_PCI_OFFSET_TO_INDEX(*ppos) - VFIO_PCI_NUM_REGIONS;
>> +	void *base = vdev->region[i].data;
>> +	loff_t pos = *ppos & VFIO_PCI_OFFSET_MASK;
>> +
>> +	if (pos >= vdev->region[i].size)
>> +		return -EINVAL;
>> +
>> +	count = min(count, (size_t)(vdev->region[i].size - pos));
>> +
>> +	if (copy_to_user(buf, base + pos, count))
>> +		return -EFAULT;
>> +
>> +	*ppos += count;
>> +
>> +	return count;
>> +}
>> +
>> +static int vfio_pci_dma_fault_mmap(struct vfio_pci_device *vdev,
>> +				   struct vfio_pci_region *region,
>> +				   struct vm_area_struct *vma)
>> +{
>> +	u64 phys_len, req_len, pgoff, req_start;
>> +	unsigned long long addr;
>> +	unsigned int index;
>> +
>> +	index = vma->vm_pgoff >> (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT);
>> +
>> +	if (vma->vm_end < vma->vm_start)
>> +		return -EINVAL;
>> +	if ((vma->vm_flags & VM_SHARED) == 0)
>> +		return -EINVAL;
>> +
>> +	phys_len = VFIO_FAULT_REGION_SIZE;
>> +
>> +	req_len = vma->vm_end - vma->vm_start;
>> +	pgoff = vma->vm_pgoff &
>> +		((1U << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);
>> +	req_start = pgoff << PAGE_SHIFT;
>> +
>> +	if (req_start + req_len > phys_len)
>> +		return -EINVAL;
>> +
>> +	addr = virt_to_phys(vdev->fault_region);
>> +	vma->vm_private_data = vdev;
>> +	vma->vm_pgoff = (addr >> PAGE_SHIFT) + pgoff;
>> +
>> +	return remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
>> +			       req_len, vma->vm_page_prot);
>> +}
>> +
>> +void vfio_pci_dma_fault_release(struct vfio_pci_device *vdev,
>> +				struct vfio_pci_region *region)
>> +{
>> +}
>> +
>> +static const struct vfio_pci_regops vfio_pci_dma_fault_regops = {
>> +	.rw		= vfio_pci_dma_fault_rw,
>> +	.mmap		= vfio_pci_dma_fault_mmap,
>> +	.release	= vfio_pci_dma_fault_release,
>> +};
>> +
>> +static int vfio_pci_init_dma_fault_region(struct vfio_pci_device *vdev)
>> +{
>> +	u32 flags = VFIO_REGION_INFO_FLAG_READ | VFIO_REGION_INFO_FLAG_WRITE |
>> +		    VFIO_REGION_INFO_FLAG_MMAP;
>> +	int ret;
>> +
>> +	spin_lock_init(&vdev->fault_queue_lock);
>> +
>> +	vdev->fault_region = kmalloc(VFIO_FAULT_REGION_SIZE, GFP_KERNEL);
>> +	if (!vdev->fault_region)
>> +		return -ENOMEM;
>> +
>> +	ret = vfio_pci_register_dev_region(vdev,
>> +		VFIO_REGION_TYPE_NESTED,
>> +		VFIO_REGION_SUBTYPE_NESTED_FAULT_REGION,
>> +		&vfio_pci_dma_fault_regops, VFIO_FAULT_REGION_SIZE,
>> +		flags, vdev->fault_region);
>> +	if (ret) {
>> +		kfree(vdev->fault_region);
>> +		return ret;
>> +	}
>> +
>> +	vdev->fault_region->header.prod = 0;
>> +	vdev->fault_region->header.cons = 0;
>> +	vdev->fault_region->header.reserved = 0;
> 
> Use kzalloc above or else we're leaking kernel memory to userspace
> anyway.
sure
> 
>> +	vdev->fault_region->header.size = VFIO_FAULT_QUEUE_SIZE;
>> +	return 0;
>> +}
>> +
>>  static int vfio_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
>>  {
>>  	struct vfio_pci_device *vdev;
>> @@ -1300,7 +1399,7 @@ static int vfio_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
>>  		pci_set_power_state(pdev, PCI_D3hot);
>>  	}
>>  
>> -	return ret;
>> +	return vfio_pci_init_dma_fault_region(vdev);
> 
> Missing lots of cleanup should this fail.  Why is this done on probe
> anyway?  This looks like something we'd do from vfio_pci_enable() and
> therefore our release callback would free fault_region rather than what
> we have below.
OK. That's fine to put in the vfio_pci_enable().
> 
>>  }
>>  
>>  static void vfio_pci_remove(struct pci_dev *pdev)
>> @@ -1315,6 +1414,7 @@ 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_region);
>>  	mutex_destroy(&vdev->ioeventfds_lock);
>>  	kfree(vdev);
>>  
>> diff --git a/drivers/vfio/pci/vfio_pci_private.h b/drivers/vfio/pci/vfio_pci_private.h
>> index 8c0009f00818..38b5d1764a26 100644
>> --- a/drivers/vfio/pci/vfio_pci_private.h
>> +++ b/drivers/vfio/pci/vfio_pci_private.h
>> @@ -120,6 +120,8 @@ struct vfio_pci_device {
>>  	int			ioeventfds_nr;
>>  	struct eventfd_ctx	*err_trigger;
>>  	struct eventfd_ctx	*req_trigger;
>> +	spinlock_t              fault_queue_lock;
>> +	struct vfio_fault_region *fault_region;
>>  	struct list_head	dummy_resources_list;
>>  	struct mutex		ioeventfds_lock;
>>  	struct list_head	ioeventfds_list;
>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>> index 352e795a93c8..b78c2c62af6d 100644
>> --- a/include/uapi/linux/vfio.h
>> +++ b/include/uapi/linux/vfio.h
>> @@ -307,6 +307,9 @@ struct vfio_region_info_cap_type {
>>  #define VFIO_REGION_TYPE_GFX                    (1)
>>  #define VFIO_REGION_SUBTYPE_GFX_EDID            (1)
>>  
>> +#define VFIO_REGION_TYPE_NESTED			(2)
>> +#define VFIO_REGION_SUBTYPE_NESTED_FAULT_REGION	(1)
>> +
>>  /**
>>   * struct vfio_region_gfx_edid - EDID region layout.
>>   *
>> @@ -697,6 +700,18 @@ struct vfio_device_ioeventfd {
>>  
>>  #define VFIO_DEVICE_IOEVENTFD		_IO(VFIO_TYPE, VFIO_BASE + 16)
>>  
>> +struct vfio_fault_region_header {
>> +	__u32	size;		/* Read-Only */
>> +	__u32	prod;		/* Read-Only */
> 
> We can't really enforce read-only if it's mmap'd.
Do we really need to? Assuming the kernel always uses
VFIO_FAULT_QUEUE_SIZE to check prod and cons indice - which is not the
case at the moment by the way :-( -s, the queue cannot
be overflown .

The header also can be checked each time the kernel fills any event in
the queue
(vfio_pci_iommu_dev_fault_handler). If inconsistent the kernel may stop
using the queue. If the user-space mangles with those RO fields, this
will break error reporting on the guest but the problem should be
confined here?


> I worry about synchronization here too, perhaps there should be a ring offset such
> that the ring can be in a separate page from the header and then sparse
> mmap support can ensure that the user access is restricted.

I was assuming a single writer and single reader lock-free circular
buffer here. My understanding was it was safe to consider concurrent
read and write. What I am missing anyway is atomic counter operations to
guarantee the indices are updated after the push/pop action as explained in
https://www.kernel.org/doc/Documentation/circular-buffers.txt. I am not
comfortable about how to enforce this on user side though.

In case I split the header and the actual buffer into 2 different
possible 64kB pages, the first one will be very scarcely used.

> wonder if there are other transports that make sense here, this almost
> feels like a vhost sort of thing.  Thanks,
Using something more sophisticated may be useful for PRI where answers
need to be provided. For the case of unrecoverable faults, I wonder
whether it is worth the pain exposing a fault region compared to the
original IOCTL approach introduced in
[RFC v2 18/20] vfio: VFIO_IOMMU_GET_FAULT_EVENTS
https://lkml.org/lkml/2018/9/18/1094

Thanks

Eric
> 
> Alex
> 
>> +	__u32	cons;
>> +	__u32	reserved;	/* must be 0 */
>> +};
>> +
>> +struct vfio_fault_region {
>> +	struct vfio_fault_region_header header;
>> +	struct iommu_fault queue[0];
>> +};
>> +
>>  /* -------- API for Type1 VFIO IOMMU -------- */
>>  
>>  /**
>