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    SubjectRe: [RFC PATCH] kvm: Use huge pages for DAX-backed files
    On 2018-10-31 at 09:49 Paolo Bonzini <pbonzini@redhat.com> wrote:
    > > On 2018-10-29 at 20:10 Dan Williams <dan.j.williams@intel.com> wrote:
    > >> The property of DAX pages that requires special coordination is the
    > >> fact that the device hosting the pages can be disabled at will. The
    > >> get_dev_pagemap() api is the interface to pin a device-pfn so that you
    > >> can safely perform a pfn_to_page() operation.
    > >>
    > >> Have the pages that kvm uses in this path already been pinned by vfio?
    >
    > No, VFIO is not involved here.
    >
    > The pages that KVM uses are never pinned. Soon after we grab them and
    > we build KVM's page table, we do put_page in mmu_set_spte (via
    > kvm_release_pfn_clean). From that point on the MMU notifier will take
    > care of invalidating SPT before the page disappears from the mm's page
    > table.

    I looked into this a little, and I think it's safe in terms of DAX's
    get_dev_pagemap() refcounts to have kvm_is_reserved_pfn() treat
    DAX pages as not reserved. kvm_is_reserved_pfn() is used before a
    pfn_to_page() call, so the concern was that the pages didn't have a DAX
    refcount.

    Many of the times that KVM looks at the PFN, it's holding the KVM
    mmu_lock, which keeps the pages in the host-side VMA. (IIUC).
    Functions like kvm_set_pfn_dirty() fall into this category.

    The other times, such as the vmexit path I mentioned before, go through
    a gfn_to_pfn call. Under the hood, those call one of the
    get_user_pages() functions during hva_to_pfn, and those do the
    right thing w.r.t. get_dev_pagemap().

    One of the other things I noticed was some places in KVM make a
    distinction between kvm_is_reserved_pfn and PageReserved:

    void kvm_set_pfn_dirty(kvm_pfn_t pfn)
    {
    if (!kvm_is_reserved_pfn(pfn)) {
    struct page *page = pfn_to_page(pfn);

    if (!PageReserved(page))
    SetPageDirty(page);
    }
    }

    I think we want to SetPageDirty for DAX, so making PageReserved be true
    for DAX seems like the way to go, or we'll need more KVM-specific
    changes. Apologies is this was discussed in the previous thread on this
    topic and is redundant.

    Thanks,

    Barret

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