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Subject[RFC PATCH 00/12] KVM: MMU: locklessly wirte-protect
Currently, when mark memslot dirty logged or get dirty page, we need to
write-protect large guest memory, it is the heavy work, especially, we need to
hold mmu-lock which is also required by vcpu to fix its page table fault and
mmu-notifier when host page is being changed. In the extreme cpu / memory used
guest, it becomes a scalability issue.

This patchset introduces a way to locklessly write-protect guest memory.

There are the challenges we meet and the ideas to resolve them.

1) How to locklessly walk rmap?
The first idea we got to prevent "desc" being freed when we are walking the
rmap is using RCU. But when vcpu runs on shadow page mode or nested mmu mode,
it updates the rmap really frequently.

So we uses SLAB_DESTROY_BY_RCU to manage "desc" instead, it allows the object
to be reused more quickly. We also store a "nulls" in the last "desc"
(desc->more) which can help us to detect whether the "desc" is moved to anther
rmap then we can re-walk the rmap if that happened. I learned this idea from

Another issue is, when a spte is deleted from the "desc", another spte in the
last "desc" will be moved to this position to replace the deleted one. If the
deleted one has been accessed and we do not access the replaced one, the
replaced one is missed when we do lockless walk.
To fix this case, we do not backward move the spte, instead, we forward move
the entry: when a spte is deleted, we move the entry in the first desc to that

2) How to locklessly access shadow page table?
It is easy if the handler is in the vcpu context, in that case we can use
walk_shadow_page_lockless_begin() and walk_shadow_page_lockless_end() that
disable interrupt to stop shadow page be freed. But we are on the ioctl context
and the paths we are optimizing for have heavy workload, disabling interrupt is
not good for the system performance.

We add a indicator into kvm struct (kvm->arch.rcu_free_shadow_page), then use
call_rcu() to free the shadow page if that indicator is set. Set/Clear the
indicator are protected by slot-lock, so it need not be atomic and does not
hurt the performance and the scalability.

3) How to locklessly write-protect guest memory?
Currently, there are two behaviors when we write-protect guest memory, one is
clearing the Writable bit on spte and the another one is dropping spte when it
points to large page. The former is easy we only need to atomicly clear a bit
but the latter is hard since we need to remove the spte from rmap. so we unify
these two behaviors that only make the spte readonly. Making large spte
readonly instead of nonpresent is also good for reducing jitter.

And we need to pay more attention on the order of making spte writable, adding
spte into rmap and setting the corresponding bit on dirty bitmap since
kvm_vm_ioctl_get_dirty_log() write-protects the spte based on the dirty bitmap,
we should ensure the writable spte can be found in rmap before the dirty bitmap
is visible. Otherwise, we cleared the dirty bitmap and failed to write-protect
the page.

Performance result
Host: CPU: Intel(R) Xeon(R) CPU X5690 @ 3.47GHz x 12
Mem: 36G

The benchmark i used and will be attached:
a) kernbench
b) migrate-perf
it emulates guest migration
c) mmtest
it repeatedly writes the memory and measures the time and is used to
generate memory access in the guest which is being migrated
d) Qemu monitor command to implement guest live migration
the script can be found in migrate-perf.

1) First, we use kernbench to benchmark the performance with non-write-protection
case to detect the possible regression:

EPT enabled: Base: 84.05 After the patch: 83.53
EPT disabled: Base: 142.57 After the patch: 141.70

No regression and the optimization may come from lazily drop large spte.

2) Benchmark the performance of get dirty page
(./migrate-perf -c 12 -m 3000 -t 20)

Base: Run 20 times, Avg time:24813809 ns.
After the patch: Run 20 times, Avg time:8371577 ns.

It improves +196%

3) There is the result of Live Migration:
3.1) Less vcpus, less memory and less dirty page generated
Guest config: MEM_SIZE=7G VCPU_NUM=6
The workload in migrated guest:
ssh -f $CLIENT "cd ~; rm -f result; nohup /home/eric/mmtest/mmtest -m 3000 -c 30 -t 60 > result &"

Live Migration time (ms) Benchmark (ns)
EPT | Baseline | 21638 | 266601028 |
+ -------------------------------+-------------+---------+
| After | 21110 +2.5% | 264966696 +0.6% |
Shadow | Baseline | 22542 | 271969284 | |
| After | 21641 +4.1% | 270485511 +0.5% |

3.2) More vcpus, more memory and less dirty page generated
Guest config: MEM_SIZE=25G VCPU_NUM=12
The workload in migrated guest:
ssh -f $CLIENT "cd ~; rm -f result; nohup /home/eric/mmtest/mmtest -m 15000 -c 30 -t 30 > result &"

Live Migration time (ms) Benchmark (ns)
EPT | Baseline | 72773 | 1278228350 |
+ -------------------------------+-------------+---------+
| After | 70516 +3.2% | 1266581587 +0.9% |
Shadow | Baseline | 74198 | 1323180090 | |
| After | 64948 +14.2% | 1299283302 +1.8% |

3.3) Less vcpus, more memory and huge dirty page generated
Guest config: MEM_SIZE=25G VCPU_NUM=6
The workload in migrated guest:
ssh -f $CLIENT "cd ~; rm -f result; nohup /home/eric/mmtest/mmtest -m 15000 -c 30 -t 200 > result &"

Live Migration time (ms) Benchmark (ns)
EPT | Baseline | 267473 | 1224657502 |
+ -------------------------------+-------------+---------+
| After | 267374 +0.03% | 1221520513 +0.6% |
Shadow | Baseline | 369999 | 1712004428 | |
| After | 335737 +10.2% | 1556065063 +10.2% |

For the case of 3.3), EPT gets small benefit, the reason is only the first
time guest writes memory need take mmu-lock to mark spte from nonpresent to
present. Other writes cost lots of time to trigger the page fault due to
write-protection which are fixed by fast page fault which need not take

Xiao Guangrong (12):
KVM: MMU: remove unused parameter
KVM: MMU: properly check last spte in fast_page_fault()
KVM: MMU: lazily drop large spte
KVM: MMU: log dirty page after marking spte writable
KVM: MMU: add spte into rmap before logging dirty page
KVM: MMU: flush tlb if the spte can be locklessly modified
KVM: MMU: redesign the algorithm of pte_list
KVM: MMU: introduce nulls desc
KVM: MMU: introduce pte-list lockless walker
KVM: MMU: allow locklessly access shadow page table out of vcpu thread
KVM: MMU: locklessly write-protect the page
KVM: MMU: clean up spte_write_protect

arch/x86/include/asm/kvm_host.h | 10 +-
arch/x86/kvm/mmu.c | 442 ++++++++++++++++++++++++++++------------
arch/x86/kvm/mmu.h | 28 +++
arch/x86/kvm/x86.c | 19 +-
4 files changed, 356 insertions(+), 143 deletions(-)


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