Re: [PATCH] vfio: add dma map/unmap support for noiommu

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, May 02, 2025 at 05:48:21PM -0700, William Tu wrote:
> Currently when using noiommu, the VFIO uAPI does not support
> VFIO_IOMMU_MAP_DMA, causing userspace VFIO program to use
> s.t like /proc/self/pagemap to get the physical address.
> The patch adds the support for dma map and unmap operation
> for noiommu.

We are not changing vfio container code in significant ways now, new
features need to go through iommufd and use an access to do this.

> +	while (remaining_pages > 0) {
> +		unsigned long pages_to_unmap = min(remaining_pages, chunk_size);
> +
> +		ret = get_user_pages_remote(mm, current_vaddr, pages_to_unmap,
> +					    FOLL_GET, pages, NULL);
> +		if (ret > 0) {

This doesn't work anyhow because there is no guarentee that the 2nd
get_user_pages will return the same things, and that is the wrong API
to use anyhow.

Jason