[Qemu-devel] [PATCH v2 3/3] KVM: MMU: fast cleanup D bit based on fast write protect

[Zhuangyanying]

From: Zhuang Yanying <address@hidden>

When live-migration with large-memory guests, vcpu may hang for a long
time while starting migration, such as 9s for 2T
(linux-5.0.0-rc2+qemu-3.1.0).
The reason is memory_global_dirty_log_start() taking too long, and the
vcpu is waiting for BQL. The page-by-page D bit clearup is the main time
consumption. I think that the idea of "KVM: MMU: fast write protect" by
xiaoguangrong, especially the function kvm_mmu_write_protect_all_pages(),
is very helpful. After a little modifcation, on his patch, can solve
this problem, 9s to 0.5s.

At the beginning of live migration, write protection is only applied to the
top-level SPTE. Then the write from vm trigger the EPT violation, with
for_each_shadow_entry write protection is performed at dirct_map.
Finally the Dirty bit of the target page(at level 1 page table) is
cleared, and the dirty page tracking is started. The page where
GPA is located is marked dirty when mmu_set_spte.

Signed-off-by: Zhuang Yanying <address@hidden>
---
 arch/x86/kvm/mmu.c     | 6 +++++-
 arch/x86/kvm/vmx/vmx.c | 5 ++---
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index effae7a..ac7a994 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3230,7 +3230,10 @@ static bool mmu_load_shadow_page(struct kvm *kvm, struct 
kvm_mmu_page *sp)
                        break;
 
                if (is_last_spte(spte, sp->role.level)) {
-                       flush |= spte_write_protect(sptep, false);
+                       if (sp->role.level == PT_PAGE_TABLE_LEVEL)
+                               flush |= spte_clear_dirty(sptep);
+                       else
+                               flush |= spte_write_protect(sptep, false);
                        continue;
                }
 
@@ -6106,6 +6109,7 @@ void kvm_mmu_write_protect_all_pages(struct kvm *kvm, 
bool write_protect)
                kvm_reload_remote_mmus(kvm);
        spin_unlock(&kvm->mmu_lock);
 }
+EXPORT_SYMBOL_GPL(kvm_mmu_write_protect_all_pages);
 
 static unsigned long
 mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index f6915f1..540ec21 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7180,14 +7180,13 @@ static void vmx_sched_in(struct kvm_vcpu *vcpu, int cpu)
 static void vmx_slot_enable_log_dirty(struct kvm *kvm,
                                     struct kvm_memory_slot *slot)
 {
-       kvm_mmu_slot_leaf_clear_dirty(kvm, slot);
-       kvm_mmu_slot_largepage_remove_write_access(kvm, slot);
+       kvm_mmu_write_protect_all_pages(kvm, true);
 }
 
 static void vmx_slot_disable_log_dirty(struct kvm *kvm,
                                       struct kvm_memory_slot *slot)
 {
-       kvm_mmu_slot_set_dirty(kvm, slot);
+       kvm_mmu_write_protect_all_pages(kvm, false);
 }
 
 static void vmx_flush_log_dirty(struct kvm *kvm)
-- 
1.8.3.1