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

[Zhuangyanying]

From: Zhuang Yanying <address@hidden>

Recently I tested live-migration with large-memory guests, find vcpu may hang 
for a long time while starting migration, such as 9s for 
2048G(linux-4.20.1+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 begining 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. Of coure, the page where GPA is located is 
marked dirty when mmu_set_spte.
A similar implementation on xen, just emt instead of write protection.

What do you think about this solution?
---
 mmu.c | 5 ++++-
 vmx.c | 3 +--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/mmu.c b/mmu.c
index b079d74..f49d316 100755
--- a/mmu.c
+++ b/mmu.c
@@ -3210,7 +3210,10 @@ 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;
                 }
 
diff --git a/vmx.c b/vmx.c
index 95784bc..7ec717f 100755
--- a/vmx.c
+++ b/vmx.c
@@ -14421,8 +14421,7 @@ 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,
-- 
1.8.3.1