Re: [RFC PATCH v3 2/8] vfio/type1: Add a page fault handler

[Shenming Lu]

On 2021/5/25 6:11, Alex Williamson wrote:
> On Fri, 21 May 2021 14:38:52 +0800
> Shenming Lu <lushenming@xxxxxxxxxx> wrote:
> 
>> On 2021/5/19 2:58, Alex Williamson wrote:
>>> On Fri, 9 Apr 2021 11:44:14 +0800
>>> Shenming Lu <lushenming@xxxxxxxxxx> wrote:
>>>   
>>>> VFIO manages the DMA mapping itself. To support IOPF (on-demand paging)
>>>> for VFIO (IOMMU capable) devices, we add a VFIO page fault handler to
>>>> serve the reported page faults from the IOMMU driver.
>>>>
>>>> Signed-off-by: Shenming Lu <lushenming@xxxxxxxxxx>
>>>> ---
>>>>  drivers/vfio/vfio_iommu_type1.c | 114 ++++++++++++++++++++++++++++++++
>>>>  1 file changed, 114 insertions(+)
>>>>
>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>> index 45cbfd4879a5..ab0ff60ee207 100644
>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>> @@ -101,6 +101,7 @@ struct vfio_dma {
>>>>  	struct task_struct	*task;
>>>>  	struct rb_root		pfn_list;	/* Ex-user pinned pfn list */
>>>>  	unsigned long		*bitmap;
>>>> +	unsigned long		*iopf_mapped_bitmap;
>>>>  };
>>>>  
>>>>  struct vfio_batch {
>>>> @@ -141,6 +142,16 @@ struct vfio_regions {
>>>>  	size_t len;
>>>>  };
>>>>  
>>>> +/* A global IOPF enabled group list */
>>>> +static struct rb_root iopf_group_list = RB_ROOT;
>>>> +static DEFINE_MUTEX(iopf_group_list_lock);
>>>> +
>>>> +struct vfio_iopf_group {
>>>> +	struct rb_node		node;
>>>> +	struct iommu_group	*iommu_group;
>>>> +	struct vfio_iommu	*iommu;
>>>> +};
>>>> +
>>>>  #define IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu)	\
>>>>  					(!list_empty(&iommu->domain_list))
>>>>  
>>>> @@ -157,6 +168,10 @@ struct vfio_regions {
>>>>  #define DIRTY_BITMAP_PAGES_MAX	 ((u64)INT_MAX)
>>>>  #define DIRTY_BITMAP_SIZE_MAX	 DIRTY_BITMAP_BYTES(DIRTY_BITMAP_PAGES_MAX)
>>>>  
>>>> +#define IOPF_MAPPED_BITMAP_GET(dma, i)	\
>>>> +			      ((dma->iopf_mapped_bitmap[(i) / BITS_PER_LONG]	\
>>>> +			       >> ((i) % BITS_PER_LONG)) & 0x1)  
>>>
>>>
>>> Can't we just use test_bit()?  
>>
>> Yeah, we can use it.
>>
>>>
>>>   
>>>> +
>>>>  #define WAITED 1
>>>>  
>>>>  static int put_pfn(unsigned long pfn, int prot);
>>>> @@ -416,6 +431,34 @@ static int vfio_iova_put_vfio_pfn(struct vfio_dma *dma, struct vfio_pfn *vpfn)
>>>>  	return ret;
>>>>  }
>>>>  
>>>> +/*
>>>> + * Helper functions for iopf_group_list
>>>> + */
>>>> +static struct vfio_iopf_group *
>>>> +vfio_find_iopf_group(struct iommu_group *iommu_group)
>>>> +{
>>>> +	struct vfio_iopf_group *iopf_group;
>>>> +	struct rb_node *node;
>>>> +
>>>> +	mutex_lock(&iopf_group_list_lock);
>>>> +
>>>> +	node = iopf_group_list.rb_node;
>>>> +
>>>> +	while (node) {
>>>> +		iopf_group = rb_entry(node, struct vfio_iopf_group, node);
>>>> +
>>>> +		if (iommu_group < iopf_group->iommu_group)
>>>> +			node = node->rb_left;
>>>> +		else if (iommu_group > iopf_group->iommu_group)
>>>> +			node = node->rb_right;
>>>> +		else
>>>> +			break;
>>>> +	}
>>>> +
>>>> +	mutex_unlock(&iopf_group_list_lock);
>>>> +	return node ? iopf_group : NULL;
>>>> +}  
>>>
>>> This looks like a pretty heavy weight operation per DMA fault.
>>>
>>> I'm also suspicious of this validity of this iopf_group after we've
>>> dropped the locking, the ordering of patches makes this very confusing.  
>>
>> My thought was to include the handling of DMA faults completely in the type1
>> backend by introducing the vfio_iopf_group struct. But it seems that introducing
>> a struct with an unknown lifecycle causes more problems...
>> I will use the path from vfio-core as in the v2 for simplicity and validity.
>>
>> Sorry for the confusing, I will reconstruct the series later. :-)
>>
>>>   
>>>> +
>>>>  static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
>>>>  {
>>>>  	struct mm_struct *mm;
>>>> @@ -3106,6 +3149,77 @@ static int vfio_iommu_type1_dirty_pages(struct vfio_iommu *iommu,
>>>>  	return -EINVAL;
>>>>  }
>>>>  
>>>> +/* VFIO I/O Page Fault handler */
>>>> +static int vfio_iommu_type1_dma_map_iopf(struct iommu_fault *fault, void *data)  
>>>   
>>> >From the comment, this seems like the IOMMU fault handler (the  
>>> construction of this series makes this difficult to follow) and
>>> eventually it handles more than DMA mapping, for example transferring
>>> faults to the device driver.  "dma_map_iopf" seems like a poorly scoped
>>> name.  
>>
>> Maybe just call it dev_fault_handler?
> 
> Better.
> 
>>>> +{
>>>> +	struct device *dev = (struct device *)data;
>>>> +	struct iommu_group *iommu_group;
>>>> +	struct vfio_iopf_group *iopf_group;
>>>> +	struct vfio_iommu *iommu;
>>>> +	struct vfio_dma *dma;
>>>> +	dma_addr_t iova = ALIGN_DOWN(fault->prm.addr, PAGE_SIZE);
>>>> +	int access_flags = 0;
>>>> +	unsigned long bit_offset, vaddr, pfn;
>>>> +	int ret;
>>>> +	enum iommu_page_response_code status = IOMMU_PAGE_RESP_INVALID;
>>>> +	struct iommu_page_response resp = {0};
>>>> +
>>>> +	if (fault->type != IOMMU_FAULT_PAGE_REQ)
>>>> +		return -EOPNOTSUPP;
>>>> +
>>>> +	iommu_group = iommu_group_get(dev);
>>>> +	if (!iommu_group)
>>>> +		return -ENODEV;
>>>> +
>>>> +	iopf_group = vfio_find_iopf_group(iommu_group);
>>>> +	iommu_group_put(iommu_group);
>>>> +	if (!iopf_group)
>>>> +		return -ENODEV;
>>>> +
>>>> +	iommu = iopf_group->iommu;
>>>> +
>>>> +	mutex_lock(&iommu->lock);  
>>>
>>> Again, I'm dubious of our ability to grab this lock from an object with
>>> an unknown lifecycle and races we might have with that group being
>>> detached or DMA unmapped.  Also, how effective is enabling IOMMU page
>>> faulting if we're serializing all faults within a container context?  
>>
>> Did you mean "efficient"?
> 
> Yes, that's more appropriate.
> 
>> I also worry about this as the mapping and unmapping of the faulting pages
>> are all with the same lock...
>> Is there a way to parallel them? Or could we have more fine grained lock
>> control?
> 
> It seems we need it; the current locking is designed for static
> mappings by the user, therefore concurrency hasn't been a priority.

I will try to implement it. :-)

> This again touches how far we want to extend type1 in the direction
> we intend to go with SVA/PASID support in IOASID.  Thanks,

Reply in the cover.

Thanks,
Shenming

> 
> Alex
> 
> .
>