[Qemu-devel] [PATCH 0/4] 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-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 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?

Xiao Guangrong (3):
  KVM: MMU: correct the behavior of mmu_spte_update_no_track
  KVM: MMU: introduce possible_writable_spte_bitmap
  KVM: MMU: introduce kvm_mmu_write_protect_all_pages

Zhuang Yanying (1):
  KVM: MMU: fast cleanup D bit based on fast write protect

 arch/x86/include/asm/kvm_host.h |  25 +++-
 arch/x86/kvm/mmu.c              | 259 ++++++++++++++++++++++++++++++++++++++--
 arch/x86/kvm/mmu.h              |   1 +
 arch/x86/kvm/paging_tmpl.h      |  13 +-
 arch/x86/kvm/vmx/vmx.c          |   3 +-
 5 files changed, 286 insertions(+), 15 deletions(-)

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1.8.3.1