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

[Alex Williamson]

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

Alex