On 09.11.18 21:39, Barret Rhoden wrote:
> This change allows KVM to map DAX-backed files made of huge pages with
> huge mappings in the EPT/TDP.
>
> DAX pages are not PageTransCompound. The existing check is trying to
> determine if the mapping for the pfn is a huge mapping or not. For
> non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound.
> For DAX, we can check the page table itself.
>
> Note that KVM already faulted in the page (or huge page) in the host's
> page table, and we hold the KVM mmu spinlock (grabbed before checking
> the mmu seq).
I wonder if the KVM mmu spinlock is enough for walking (not KVM
exclusive) host page tables. Can you elaborate?
>
> Signed-off-by: Barret Rhoden <brho@xxxxxxxxxx>
> ---
> arch/x86/kvm/mmu.c | 34 ++++++++++++++++++++++++++++++++--
> 1 file changed, 32 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index cf5f572f2305..2df8c459dc6a 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -3152,6 +3152,36 @@ static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
> return -EFAULT;
> }
>
> +static bool pfn_is_huge_mapped(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn)
> +{
> + struct page *page = pfn_to_page(pfn);
> + unsigned long hva, map_shift;
> +
> + if (!is_zone_device_page(page))
> + return PageTransCompoundMap(page);
> +
> + /*
> + * DAX pages do not use compound pages. The page should have already
> + * been mapped into the host-side page table during try_async_pf(), so
> + * we can check the page tables directly.
> + */
> + hva = gfn_to_hva(kvm, gfn);
> + if (kvm_is_error_hva(hva))
> + return false;
> +
> + /*
> + * Our caller grabbed the KVM mmu_lock with a successful
> + * mmu_notifier_retry, so we're safe to walk the page table.
> + */
> + map_shift = dev_pagemap_mapping_shift(hva, current->mm);
You could get rid of that local variable map_shift.
> + switch (map_shift) {
> + case PMD_SHIFT:
> + case PUD_SIZE:
> + return true;
> + }
> + return false;
> +}
> +
> static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
> gfn_t *gfnp, kvm_pfn_t *pfnp,
> int *levelp)
> @@ -3168,7 +3198,7 @@ static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
> */
> if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn) &&
> level == PT_PAGE_TABLE_LEVEL &&
> - PageTransCompoundMap(pfn_to_page(pfn)) &&
> + pfn_is_huge_mapped(vcpu->kvm, gfn, pfn) &&
> !mmu_gfn_lpage_is_disallowed(vcpu, gfn, PT_DIRECTORY_LEVEL)) {
> unsigned long mask;
> /*
> @@ -5678,7 +5708,7 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
> */
> if (sp->role.direct &&
> !kvm_is_reserved_pfn(pfn) &&
> - PageTransCompoundMap(pfn_to_page(pfn))) {
> + pfn_is_huge_mapped(kvm, sp->gfn, pfn)) {
> pte_list_remove(rmap_head, sptep);
> need_tlb_flush = 1;
> goto restart;
>
This looks surprisingly simple to me :)
--
Thanks,
David / dhildenb
