Re: [RFC PATCH v12 14/33] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

[Yan Zhao]

On Wed, Sep 13, 2023 at 06:55:12PM -0700, Sean Christopherson wrote:
> +static struct folio *kvm_gmem_get_folio(struct file *file, pgoff_t index)
> +{
> +     struct folio *folio;
> +
> +     /* TODO: Support huge pages. */
> +     folio = filemap_grab_folio(file->f_mapping, index);
> +     if (IS_ERR_OR_NULL(folio))
> +             return NULL;
> +
> +     /*
> +      * Use the up-to-date flag to track whether or not the memory has been
> +      * zeroed before being handed off to the guest.  There is no backing
> +      * storage for the memory, so the folio will remain up-to-date until
> +      * it's removed.
> +      *
> +      * TODO: Skip clearing pages when trusted firmware will do it when
> +      * assigning memory to the guest.
> +      */
> +     if (!folio_test_uptodate(folio)) {
> +             unsigned long nr_pages = folio_nr_pages(folio);
> +             unsigned long i;
> +
> +             for (i = 0; i < nr_pages; i++)
> +                     clear_highpage(folio_page(folio, i));
> +
> +             folio_mark_uptodate(folio);
> +     }
> +
> +     /*
> +      * Ignore accessed, referenced, and dirty flags.  The memory is
> +      * unevictable and there is no storage to write back to.
> +      */
> +     return folio;
> +}
If VFIO wants to map a private page, is it required to call this function for 
PFN?