Re: [PATCH 1/3] vfio/type1: Support faulting PFNMAP vmas

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 1 May 2020 20:50:33 -0300
Jason Gunthorpe <jgg@ziepe.ca> wrote:

> On Fri, May 01, 2020 at 03:39:08PM -0600, Alex Williamson wrote:
> > With conversion to follow_pfn(), DMA mapping a PFNMAP range depends on
> > the range being faulted into the vma.  Add support to manually provide
> > that, in the same way as done on KVM with hva_to_pfn_remapped().
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> >  drivers/vfio/vfio_iommu_type1.c |   36 +++++++++++++++++++++++++++++++++---
> >  1 file changed, 33 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index cc1d64765ce7..4a4cb7cd86b2 100644
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -317,6 +317,32 @@ static int put_pfn(unsigned long pfn, int prot)
> >  	return 0;
> >  }
> >  
> > +static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
> > +			    unsigned long vaddr, unsigned long *pfn,
> > +			    bool write_fault)
> > +{
> > +	int ret;
> > +
> > +	ret = follow_pfn(vma, vaddr, pfn);
> > +	if (ret) {
> > +		bool unlocked = false;
> > +
> > +		ret = fixup_user_fault(NULL, mm, vaddr,
> > +				       FAULT_FLAG_REMOTE |
> > +				       (write_fault ?  FAULT_FLAG_WRITE : 0),
> > +				       &unlocked);
> > +		if (unlocked)
> > +			return -EAGAIN;
> > +
> > +		if (ret)
> > +			return ret;
> > +
> > +		ret = follow_pfn(vma, vaddr, pfn);
> > +	}
> > +
> > +	return ret;
> > +}
> > +
> >  static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
> >  			 int prot, unsigned long *pfn)
> >  {
> > @@ -339,12 +365,16 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
> >  
> >  	vaddr = untagged_addr(vaddr);
> >  
> > +retry:
> >  	vma = find_vma_intersection(mm, vaddr, vaddr + 1);
> >  
> >  	if (vma && vma->vm_flags & VM_PFNMAP) {
> > -		if (!follow_pfn(vma, vaddr, pfn) &&
> > -		    is_invalid_reserved_pfn(*pfn))
> > -			ret = 0;
> > +		ret = follow_fault_pfn(vma, mm, vaddr, pfn, prot & IOMMU_WRITE);
> > +		if (ret == -EAGAIN)
> > +			goto retry;
> > +
> > +		if (!ret && !is_invalid_reserved_pfn(*pfn))
> > +			ret = -EFAULT;  
> 
> I suggest checking vma->vm_ops == &vfio_pci_mmap_ops and adding a
> comment that this is racy and needs to be fixed up. The ops check
> makes this only used by other vfio bars and should prevent some
> abuses of this hacky thing

We can't do that, vfio-pci is only one bus driver within the vfio
ecosystem.

> However, I wonder if this chould just link itself into the
> vma->private data so that when the vfio that owns the bar goes away,
> so does the iommu mapping?

I don't really see why we wouldn't use mmu notifiers so that the vfio
iommu backend and vfio bus driver remain independent.

> I feel like this patch set is not complete unless it also handles the
> shootdown of this path too?

It would be nice to solve both issues and I'll start working on the mmu
notifier side of things, but this series does solve a real issue on
its own and we're not changing the iommu mapping behavior here.  Thanks,

Alex