On Tue, Feb 18, 2025 at 10:09:59PM +1100, Alexey Kardashevskiy wrote:
> CoCo VMs get their private memory allocated from guest_memfd
> ("gmemfd") which is a KVM facility similar to memfd.
> At the moment gmemfds cannot mmap() so the usual GUP API does
> not work on these as expected.
>
> Use the existing IOMMU_IOAS_MAP_FILE API to allow mapping from
> fd + offset. Detect the gmemfd case in pfn_reader_user_pin() and
> simplified mapping.
>
> The long term plan is to ditch this workaround and follow
> the usual memfd path.
How is that possible though?
> +static struct folio *guest_memfd_get_pfn(struct file *file, unsigned long index,
> + unsigned long *pfn, int *max_order)
> +{
> + struct folio *folio;
> + int ret = 0;
> +
> + folio = filemap_grab_folio(file_inode(file)->i_mapping, index);
> +
> + if (IS_ERR(folio))
> + return folio;
> +
> + if (folio_test_hwpoison(folio)) {
> + folio_unlock(folio);
> + folio_put(folio);
> + return ERR_PTR(-EHWPOISON);
> + }
> +
> + *pfn = folio_pfn(folio) + (index & (folio_nr_pages(folio) - 1));
> + if (!max_order)
> + goto unlock_exit;
> +
> + /* Refs for unpin_user_page_range_dirty_lock->gup_put_folio(FOLL_PIN) */
> + ret = folio_add_pins(folio, 1);
> + folio_put(folio); /* Drop ref from filemap_grab_folio */
> +
> +unlock_exit:
> + folio_unlock(folio);
> + if (ret)
> + folio = ERR_PTR(ret);
> +
> + return folio;
> +}
Connecting iommufd to guestmemfd through the FD is broadly the right
idea, but I'm not sure this matches the design of guestmemfd regarding
pinnability. IIRC they were adamant that the pages would not be
pinned..
folio_add_pins() just prevents the folio from being freed, it doesn't
prevent the guestmemfd code from messing with the filemap.
You should separate this from the rest of the series and discuss it
directly with the guestmemfd maintainers.
As I understood it the requirement here is to have some kind of
invalidation callback so that iommufd can drop mappings, but I don't
really know and AFAIK AMD is special in wanting private pages mapped
to the hypervisor iommu..
Jason
