KVM: Avoid checking huge page mappings in get_dirty_log()

Dropped such mappings when we enabled dirty logging and we will never
create new ones until we stop the logging.

For this we introduce a new function which can be used to write protect
a range of PT level pages: although we do not need to care about a range
of pages at this point, the following patch will need this feature to
optimize the write protection of many pages.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>

3 files changed, 36 insertions, 17 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e216ba066e79..f624ca72ea24 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -712,8 +712,9 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
 
 int kvm_mmu_reset_context(struct kvm_vcpu *vcpu);
 void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot);
-int kvm_mmu_rmap_write_protect(struct kvm *kvm, u64 gfn,
-                               struct kvm_memory_slot *slot);
+void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
+                                     struct kvm_memory_slot *slot,
+                                     gfn_t gfn_offset, unsigned long mask);
 void kvm_mmu_zap_all(struct kvm *kvm);
 unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
 void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int kvm_nr_mmu_pages);
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index c8b5694d1a48..dc5f2459db6c 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1037,27 +1037,47 @@ static int __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level
         return write_protected;
 }
 
-int kvm_mmu_rmap_write_protect(struct kvm *kvm, u64 gfn,
-                               struct kvm_memory_slot *slot)
+/**
+ * kvm_mmu_write_protect_pt_masked - write protect selected PT level pages
+ * @kvm: kvm instance
+ * @slot: slot to protect
+ * @gfn_offset: start of the BITS_PER_LONG pages we care about
+ * @mask: indicates which pages we should protect
+ *
+ * Used when we do not need to care about huge page mappings: e.g. during dirty
+ * logging we do not have any such mappings.
+ */
+void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
+                                     struct kvm_memory_slot *slot,
+                                     gfn_t gfn_offset, unsigned long mask)
 {
         unsigned long *rmapp;
-        int i, write_protected = 0;
 
-        for (i = PT_PAGE_TABLE_LEVEL;
-             i < PT_PAGE_TABLE_LEVEL + KVM_NR_PAGE_SIZES; ++i) {
-                rmapp = __gfn_to_rmap(gfn, i, slot);
-                write_protected |= __rmap_write_protect(kvm, rmapp, i);
-        }
+        while (mask) {
+                rmapp = &slot->rmap[gfn_offset + __ffs(mask)];
+                __rmap_write_protect(kvm, rmapp, PT_PAGE_TABLE_LEVEL);
 
-        return write_protected;
+                /* clear the first set bit */
+                mask &= mask - 1;
+        }
 }
 
 static int rmap_write_protect(struct kvm *kvm, u64 gfn)
 {
         struct kvm_memory_slot *slot;
+        unsigned long *rmapp;
+        int i;
+        int write_protected = 0;
 
         slot = gfn_to_memslot(kvm, gfn);
-        return kvm_mmu_rmap_write_protect(kvm, gfn, slot);
+
+        for (i = PT_PAGE_TABLE_LEVEL;
+             i < PT_PAGE_TABLE_LEVEL + KVM_NR_PAGE_SIZES; ++i) {
+                rmapp = __gfn_to_rmap(gfn, i, slot);
+                write_protected |= __rmap_write_protect(kvm, rmapp, i);
+        }
+
+        return write_protected;
 }
 
 static int kvm_unmap_rmapp(struct kvm *kvm, unsigned long *rmapp,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 99b738028fc0..813ebf1e55a0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3095,13 +3095,11 @@ static void write_protect_slot(struct kvm *kvm,
 
         /* Not many dirty pages compared to # of shadow pages. */
         if (nr_dirty_pages < kvm->arch.n_used_mmu_pages) {
-                unsigned long gfn_offset;
+                gfn_t offset;
 
-                for_each_set_bit(gfn_offset, dirty_bitmap, memslot->npages) {
-                        unsigned long gfn = memslot->base_gfn + gfn_offset;
+                for_each_set_bit(offset, dirty_bitmap, memslot->npages)
+                        kvm_mmu_write_protect_pt_masked(kvm, memslot, offset, 1);
 
-                        kvm_mmu_rmap_write_protect(kvm, gfn, memslot);
-                }
                 kvm_flush_remote_tlbs(kvm);
         } else
                 kvm_mmu_slot_remove_write_access(kvm, memslot->id);