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    SubjectRe: [PATCH V5 4/4] kvm: add a check if pfn is from NVDIMM pmem.
    On Fri, Sep 7, 2018 at 2:25 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 identify 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>
    > Acked-by: Pankaj Gupta <pagupta@redhat.com>
    > ---
    > virt/kvm/kvm_main.c | 16 ++++++++++++++--
    > 1 file changed, 14 insertions(+), 2 deletions(-)
    >
    > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
    > index c44c406..9c49634 100644
    > --- a/virt/kvm/kvm_main.c
    > +++ b/virt/kvm/kvm_main.c
    > @@ -147,8 +147,20 @@ __weak void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
    >
    > 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);
    > +
    > + /*
    > + * For device specific memory space, there is a case
    > + * which we need pass MEMORY_DEVICE_FS[DEV]_DAX pages
    > + * to kvm, these pages marked reserved flag as it is a
    > + * zone device memory, we need to identify these pages
    > + * and let kvm treat these as normal pages
    > + */
    > + return PageReserved(page) && !is_dax_page(page);

    Should we consider just not setting PageReserved for
    devm_memremap_pages()? Perhaps kvm is not be the only component making
    these assumptions about this flag?

    Why is MEMORY_DEVICE_PUBLIC memory specifically excluded?

    This has less to do with "dax" pages and more to do with
    devm_memremap_pages() established ranges. P2PDMA is another producer
    of these pages. If either MEMORY_DEVICE_PUBLIC or P2PDMA pages can be
    used in these kvm paths then I think this points to consider clearing
    the Reserved flag.

    That said I haven't audited all the locations that test PageReserved().

    Sorry for not responding sooner I was on extended leave.

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