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Subject[PATCH 14/14] KVM: x86/mmu: Use huge pages for DAX-backed files
Date
Walk the host page tables to identify hugepage mappings for ZONE_DEVICE
pfns, i.e. DAX pages. Explicitly query kvm_is_zone_device_pfn() when
deciding whether or not to bother walking the host page tables, as DAX
pages do not set up the head/tail infrastructure, i.e. will return false
for PageCompound() even when using huge pages.

Zap ZONE_DEVICE sptes when disabling dirty logging, e.g. if live
migration fails, to allow KVM to rebuild large pages for DAX-based
mappings. Presumably DAX favors large pages, and worst case scenario is
a minor performance hit as KVM will need to re-fault all DAX-based
pages.

Suggested-by: Barret Rhoden <brho@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jason Zeng <jason.zeng@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Liran Alon <liran.alon@oracle.com>
Cc: linux-nvdimm <linux-nvdimm@lists.01.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
arch/x86/kvm/mmu/mmu.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 1e4e0ac169a7..324e1919722f 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3250,7 +3250,7 @@ static int host_pfn_mapping_level(struct kvm_vcpu *vcpu, gfn_t gfn,
PT_DIRECTORY_LEVEL != (int)PG_LEVEL_2M ||
PT_PDPE_LEVEL != (int)PG_LEVEL_1G);

- if (!PageCompound(pfn_to_page(pfn)))
+ if (!PageCompound(pfn_to_page(pfn)) && !kvm_is_zone_device_pfn(pfn))
return PT_PAGE_TABLE_LEVEL;

/*
@@ -3282,8 +3282,7 @@ static int kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, gfn_t gfn,
if (unlikely(max_level == PT_PAGE_TABLE_LEVEL))
return PT_PAGE_TABLE_LEVEL;

- if (is_error_noslot_pfn(pfn) || kvm_is_reserved_pfn(pfn) ||
- kvm_is_zone_device_pfn(pfn))
+ if (is_error_noslot_pfn(pfn) || kvm_is_reserved_pfn(pfn))
return PT_PAGE_TABLE_LEVEL;

slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
@@ -5910,8 +5909,8 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
* mapping if the indirect sp has level = 1.
*/
if (sp->role.direct && !kvm_is_reserved_pfn(pfn) &&
- !kvm_is_zone_device_pfn(pfn) &&
- PageCompound(pfn_to_page(pfn))) {
+ (kvm_is_zone_device_pfn(pfn) ||
+ PageCompound(pfn_to_page(pfn)))) {
pte_list_remove(rmap_head, sptep);

if (kvm_available_flush_tlb_with_range())
--
2.24.1
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