Re: [PATCH 3/3] kvm: add a function to check if page is from NVDIMM pmem.

[Dan Williams <dan.j.williams@intel.com>]

[ adding Jerome ]

On Wed, Jul 4, 2018 at 8:30 AM, Zhang Yi <yi.z.zhang@linux.intel.com> wrote:
> For device specific memory space, when we move these area of pfn to
> memory zone, we will set the page reserved flag at that time, some of
> these reserved for device mmio, and some of these are not, such as
> NVDIMM pmem.
>
> Now, we map these dev_dax or fs_dax pages to kvm for DIMM/NVDIMM
> backend, since these pages are reserved. the check of
> kvm_is_reserved_pfn() misconceives those pages as MMIO. Therefor, we
> introduce 2 page map types, MEMORY_DEVICE_FS_DAX/MEMORY_DEVICE_DEV_DAX,
> to indentify these pages are from NVDIMM pmem. and let kvm treat these
> as normal pages.
>
> Without this patch, Many operations will be missed due to this
> mistreatment to pmem pages. For example, a page may not have chance to
> be unpinned for KVM guest(in kvm_release_pfn_clean); not able to be
> marked as dirty/accessed(in kvm_set_pfn_dirty/accessed) etc.
>
> Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> Signed-off-by: Zhang Yu <yu.c.zhang@linux.intel.com>
> ---
>  virt/kvm/kvm_main.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index afb2e6e..1365d18 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -140,10 +140,23 @@ __weak void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
>  {
>  }
>
> +static bool kvm_is_nd_pfn(kvm_pfn_t pfn)
> +{
> +       struct page *page = pfn_to_page(pfn);
> +
> +       return is_zone_device_page(page) &&
> +               ((page->pgmap->type == MEMORY_DEVICE_FS_DAX) ||
> +                (page->pgmap->type == MEMORY_DEVICE_DEV_DAX));

Jerome, might there be any use case to pass MEMORY_DEVICE_PUBLIC
memory to a guest vm?

> +}
> +
>  bool kvm_is_reserved_pfn(kvm_pfn_t pfn)
>  {
> -       if (pfn_valid(pfn))
> -               return PageReserved(pfn_to_page(pfn));
> +       struct page *page;
> +
> +       if (pfn_valid(pfn)) {
> +               page = pfn_to_page(pfn);
> +               return kvm_is_nd_pfn(pfn) ? false : PageReserved(page);
> +       }
>
>         return true;
>  }
> --
> 2.7.4
>