Re: [PATCH 2/3] vfio-pci: Fault mmaps to enable vma tracking

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, May 01, 2020 at 03:39:19PM -0600, Alex Williamson wrote:
> Rather than calling remap_pfn_range() when a region is mmap'd, setup
> a vm_ops handler to support dynamic faulting of the range on access.
> This allows us to manage a list of vmas actively mapping the area that
> we can later use to invalidate those mappings.  The open callback
> invalidates the vma range so that all tracking is inserted in the
> fault handler and removed in the close handler.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>  drivers/vfio/pci/vfio_pci.c         |   76 ++++++++++++++++++++++++++++++++++-
>  drivers/vfio/pci/vfio_pci_private.h |    7 +++
>  2 files changed, 81 insertions(+), 2 deletions(-)

> +static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
> +{
> +	struct vm_area_struct *vma = vmf->vma;
> +	struct vfio_pci_device *vdev = vma->vm_private_data;
> +
> +	if (vfio_pci_add_vma(vdev, vma))
> +		return VM_FAULT_OOM;
> +
> +	if (remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
> +			    vma->vm_end - vma->vm_start, vma->vm_page_prot))
> +		return VM_FAULT_SIGBUS;
> +
> +	return VM_FAULT_NOPAGE;
> +}
> +
> +static const struct vm_operations_struct vfio_pci_mmap_ops = {
> +	.open = vfio_pci_mmap_open,
> +	.close = vfio_pci_mmap_close,
> +	.fault = vfio_pci_mmap_fault,
> +};
> +
>  static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
>  {
>  	struct vfio_pci_device *vdev = device_data;
> @@ -1357,8 +1421,14 @@ static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
>  	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>  	vma->vm_pgoff = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;
>  
> -	return remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
> -			       req_len, vma->vm_page_prot);
> +	/*
> +	 * See remap_pfn_range(), called from vfio_pci_fault() but we can't
> +	 * change vm_flags within the fault handler.  Set them now.
> +	 */
> +	vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
> +	vma->vm_ops = &vfio_pci_mmap_ops;

Perhaps do the vfio_pci_add_vma & remap_pfn_range combo here if the
BAR is activated ? That way a fully populated BAR is presented in the
common case and avoids taking a fault path?

But it does seem OK as is

Jason