Marcelo do you feel your comments are addressed in patches 3 and 5 of this series? On Thu, Sep 05, 2013 at 06:29:03PM +0800, Xiao Guangrong wrote: > Changelog v2: > > - the changes from Gleb's review: > 1) fix calculating the number of spte in the pte_list_add() > 2) set iter->desc to NULL if meet a nulls desc to cleanup the code of > rmap_get_next() > 3) fix hlist corruption due to accessing sp->hlish out of mmu-lock > 4) use rcu functions to access the rcu protected pointer > 5) spte will be missed in lockless walker if the spte is moved in a desc > (remove a spte from the rmap using only one desc). Fix it by bottom-up > walking the desc > > - the changes from Paolo's review > 1) make the order and memory barriers between update spte / add spte into > rmap and dirty-log more clear > > - the changes from Marcelo's review: > 1) let fast page fault only fix the spts on the last level (level = 1) > 2) improve some changelogs and comments > > - the changes from Takuya's review: > move the patch "flush tlb if the spte can be locklessly modified" forward > to make it's more easily merged > > Thank all of you very much for your time and patience on this patchset! > > Since we use rcu_assign_pointer() to update the points in desc even if dirty > log is disabled, i have measured the performance: > Host: Intel(R) Xeon(R) CPU X5690 @ 3.47GHz * 12 + 36G memory > > - migrate-perf (benchmark the time of get-dirty-log) > before: Run 10 times, Avg time:9009483 ns. > after: Run 10 times, Avg time:4807343 ns. > > - kerbench > Guest: 12 VCPUs + 8G memory > before: > EPT is enabled: > # cat 09-05-origin-ept | grep real > real 85.58 > real 83.47 > real 82.95 > > EPT is disabled: > # cat 09-05-origin-shadow | grep real > real 138.77 > real 138.99 > real 139.55 > > after: > EPT is enabled: > # cat 09-05-lockless-ept | grep real > real 83.40 > real 82.81 > real 83.39 > > EPT is disabled: > # cat 09-05-lockless-shadow | grep real > real 138.91 > real 139.71 > real 138.94 > > No performance regression! > > > > Background > ========== > 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. > > Idea > ========== > 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 > nulls-list. > > 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 > position. > > 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 > ==================== > The performance result and the benchmark can be found at: > http://permalink.gmane.org/gmane.linux.kernel/1534876 > > Xiao Guangrong (15): > KVM: MMU: fix the count of spte number > KVM: MMU: properly check last spte in fast_page_fault() > KVM: MMU: lazily drop large spte > KVM: MMU: flush tlb if the spte can be locklessly modified > KVM: MMU: flush tlb out of mmu lock when write-protect the sptes > KVM: MMU: update spte and add it into rmap before dirty log > KVM: MMU: redesign the algorithm of pte_list > KVM: MMU: introduce nulls desc > KVM: MMU: introduce pte-list lockless walker > KVM: MMU: initialize the pointers in pte_list_desc properly > KVM: MMU: reintroduce kvm_mmu_isolate_page() > 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 > KVM: MMU: use rcu functions to access the pointer > > arch/x86/include/asm/kvm_host.h | 10 +- > arch/x86/kvm/mmu.c | 566 ++++++++++++++++++++++++++++++---------- > arch/x86/kvm/mmu.h | 28 ++ > arch/x86/kvm/x86.c | 34 ++- > 4 files changed, 491 insertions(+), 147 deletions(-) > > -- > 1.8.1.4 -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
