[Qemu-devel] [PATCH 4/4] 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. 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.

Signed-off-by: Zhuang Yanying <address@hidden>
---
 arch/x86/kvm/mmu.c     | 8 +++++---
 arch/x86/kvm/vmx/vmx.c | 3 +--
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 047b897..a18bcc0 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3257,7 +3257,10 @@ 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;
                }
 
@@ -6114,7 +6117,6 @@ void kvm_mmu_write_protect_all_pages(struct kvm *kvm, 
bool write_protect)
 {
        u64 wp_all_indicator, kvm_wp_all_gen;
 
-       mutex_lock(&kvm->slots_lock);
        wp_all_indicator = get_write_protect_all_indicator(kvm);
        kvm_wp_all_gen = get_write_protect_all_gen(wp_all_indicator);
 
@@ -6134,8 +6136,8 @@ void kvm_mmu_write_protect_all_pages(struct kvm *kvm, 
bool write_protect)
         */
        if (write_protect)
                kvm_reload_remote_mmus(kvm);
-       mutex_unlock(&kvm->slots_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..5236a07 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7180,8 +7180,7 @@ 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