On Thu, Jan 17, 2019 at 01:55:31PM +0000, Zhuangyanying wrote:
> From: Zhuang Yanying <ann.zhuangyany...@huawei.com>
>
> 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 <ann.zhuangyany...@huawei.com>
> ---
> 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);
Why is the lock removed? And why was it added in the first place?
> }
> +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);
What's the purpose of having @write_protect if
kvm_mmu_write_protect_all_pages() is only ever called to enable
protection? If there's no known scenario where write protection is
explicitly disabled then there's no need for WP_ALL_ENABLE_MASK, i.e. a
non-zero generation would indicate write protection is enabled. That'd
simplify the code and clean up the atomic usage.
> }
>
> static void vmx_slot_disable_log_dirty(struct kvm *kvm,
> --
> 1.8.3.1
>
>
