Re: [Xen-devel] [PATCH v1 04/10] vfio/type1: Prepare is_invalid_reserved_pfn() for PG_reserved changes

[Dan Williams <dan.j.williams@xxxxxxxxx>]

On Thu, Oct 24, 2019 at 5:12 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> Right now, ZONE_DEVICE memory is always set PG_reserved. We want to
> change that.
>
> KVM has this weird use case that you can map anything from /dev/mem
> into the guest. pfn_valid() is not a reliable check whether the memmap
> was initialized and can be touched. pfn_to_online_page() makes sure
> that we have an initialized memmap (and don't have ZONE_DEVICE memory).
>
> Rewrite is_invalid_reserved_pfn() similar to kvm_is_reserved_pfn() to make
> sure the function produces the same result once we stop setting ZONE_DEVICE
> pages PG_reserved.
>
> Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Cc: Cornelia Huck <cohuck@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 2ada8e6cdb88..f8ce8c408ba8 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -299,9 +299,15 @@ static int vfio_lock_acct(struct vfio_dma *dma, long 
> npage, bool async)
>   */
>  static bool is_invalid_reserved_pfn(unsigned long pfn)
>  {
> -       if (pfn_valid(pfn))
> -               return PageReserved(pfn_to_page(pfn));
> +       struct page *page = pfn_to_online_page(pfn);

Ugh, I just realized this is not a safe conversion until
pfn_to_online_page() is moved over to subsection granularity. As it
stands it will return true for any ZONE_DEVICE pages that share a
section with boot memory.

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