Re: [PATCH v3 13/15] KVM: MMU: locklessly write-protect the page

Gleb Natapov
2013-10-24T09:17:40Z

On Wed, Oct 23, 2013 at 09:29:31PM +0800, Xiao Guangrong wrote:
> 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 patch introduces a way to locklessly write-protect guest memory
>
> Now, lockless rmap walk, lockless shadow page table access and lockless
> spte wirte-protection are ready, it is the time to implements page
> write-protection out of mmu-lock
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> ---
> arch/x86/include/asm/kvm_host.h | 4 ---
> arch/x86/kvm/mmu.c | 59 ++++++++++++++++++++++++++++++-----------
> arch/x86/kvm/mmu.h | 6 +++++
> arch/x86/kvm/x86.c | 11 ++++----
> 4 files changed, 55 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index df9ae10..cdb6f29 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -793,10 +793,6 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
> u64 dirty_mask, u64 nx_mask, u64 x_mask);
>
> void kvm_mmu_reset_context(struct kvm_vcpu *vcpu);
> -void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot);
> -void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
> - struct kvm_memory_slot *slot,
> - gfn_t gfn_offset, unsigned long mask);
> void kvm_mmu_zap_all(struct kvm *kvm);
> void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm);
> unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 8b96d96..d82bbec 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1386,8 +1386,37 @@ static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
> return flush;
> }
>
> -/**
> - * kvm_mmu_write_protect_pt_masked - write protect selected PT level pages
> +static void __rmap_write_protect_lockless(u64 *sptep)
> +{
> + u64 spte;
> +
> +retry:
> + /*
> + * Note we may partly read the sptep on 32bit host, however, we
> + * allow this case because:
> + * - we do not access the page got from the sptep.
> + * - cmpxchg64 can detect that case and avoid setting a wrong value
> + * to the sptep.
> + */
> + spte = *rcu_dereference(sptep);
> + if (unlikely(!is_last_spte(spte) || !is_writable_pte(spte)))
is_last_spte gets two parameters.

> + return;
> +
> + if (likely(cmpxchg64(sptep, spte, spte & ~PT_WRITABLE_MASK) == spte))
> + return;
> +
> + goto retry;
> +}
> +
> +static void rmap_write_protect_lockless(unsigned long *rmapp)
> +{
> + pte_list_walk_lockless(rmapp, __rmap_write_protect_lockless);
> +}
> +
> +/*
> + * kvm_mmu_write_protect_pt_masked_lockless - write protect selected PT level
> + * pages out of mmu-lock.
> + *
> * @kvm: kvm instance
> * @slot: slot to protect
> * @gfn_offset: start of the BITS_PER_LONG pages we care about
> @@ -1396,16 +1425,17 @@ static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
> * Used when we do not need to care about huge page mappings: e.g. during dirty
> * logging we do not have any such mappings.
> */
> -void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
> - struct kvm_memory_slot *slot,
> - gfn_t gfn_offset, unsigned long mask)
> +void
> +kvm_mmu_write_protect_pt_masked_lockless(struct kvm *kvm,
> + struct kvm_memory_slot *slot,
> + gfn_t gfn_offset, unsigned long mask)
> {
> unsigned long *rmapp;
>
> while (mask) {
> rmapp = __gfn_to_rmap(slot->base_gfn + gfn_offset + __ffs(mask),
> PT_PAGE_TABLE_LEVEL, slot);
> - __rmap_write_protect(kvm, rmapp, false);
> + rmap_write_protect_lockless(rmapp);
>
> /* clear the first set bit */
> mask &= mask - 1;
> @@ -4477,7 +4507,7 @@ void kvm_mmu_setup(struct kvm_vcpu *vcpu)
> init_kvm_mmu(vcpu);
> }
>
> -void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
> +void kvm_mmu_slot_remove_write_access_lockless(struct kvm *kvm, int slot)
> {
> struct kvm_memory_slot *memslot;
> gfn_t last_gfn;
> @@ -4486,8 +4516,7 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
> memslot = id_to_memslot(kvm->memslots, slot);
> last_gfn = memslot->base_gfn + memslot->npages - 1;
>
> - spin_lock(&kvm->mmu_lock);
> -
> + rcu_read_lock();
> for (i = PT_PAGE_TABLE_LEVEL;
> i < PT_PAGE_TABLE_LEVEL + KVM_NR_PAGE_SIZES; ++i) {
> unsigned long *rmapp;
> @@ -4497,15 +4526,15 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
> last_index = gfn_to_index(last_gfn, memslot->base_gfn, i);
>
> for (index = 0; index <= last_index; ++index, ++rmapp) {
> - if (*rmapp)
> - __rmap_write_protect(kvm, rmapp, false);
> + rmap_write_protect_lockless(rmapp);
>
> - if (need_resched() || spin_needbreak(&kvm->mmu_lock))
> - cond_resched_lock(&kvm->mmu_lock);
> + if (need_resched()) {
> + rcu_read_lock();
> + rcu_read_unlock();
> + }
> }
> }
> -
> - spin_unlock(&kvm->mmu_lock);
> + rcu_read_unlock();
>
> /*
> * We can flush all the TLBs out of the mmu lock without TLB
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 2926152..33f313b 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -117,4 +117,10 @@ static inline bool permission_fault(struct kvm_mmu *mmu, unsigned pte_access,
> }
>
> void kvm_mmu_invalidate_zap_all_pages(struct kvm *kvm);
> +
> +void kvm_mmu_slot_remove_write_access_lockless(struct kvm *kvm, int slot);
> +void
> +kvm_mmu_write_protect_pt_masked_lockless(struct kvm *kvm,
> + struct kvm_memory_slot *slot,
> + gfn_t gfn_offset, unsigned long mask);
> #endif
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4ac3a27..c6233e1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3554,8 +3554,7 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
> dirty_bitmap_buffer = dirty_bitmap + n / sizeof(long);
> memset(dirty_bitmap_buffer, 0, n);
>
> - spin_lock(&kvm->mmu_lock);
> -
> + rcu_read_lock();
> for (i = 0; i < n / sizeof(long); i++) {
> unsigned long mask;
> gfn_t offset;
> @@ -3579,10 +3578,10 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
> dirty_bitmap_buffer[i] = mask;
>
> offset = i * BITS_PER_LONG;
> - kvm_mmu_write_protect_pt_masked(kvm, memslot, offset, mask);
> + kvm_mmu_write_protect_pt_masked_lockless(kvm, memslot,
> + offset, mask);
> }
> -
> - spin_unlock(&kvm->mmu_lock);
> + rcu_read_unlock();
>
> /*
> * All the TLBs can be flushed out of mmu lock, see the comments in
> @@ -7246,7 +7245,7 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> * See the comments in fast_page_fault().
> */
> if ((change != KVM_MR_DELETE) && (mem->flags & KVM_MEM_LOG_DIRTY_PAGES))
> - kvm_mmu_slot_remove_write_access(kvm, mem->slot);
> + kvm_mmu_slot_remove_write_access_lockless(kvm, mem->slot);
> }
>
> void kvm_arch_flush_shadow_all(struct kvm *kvm)
> --
> 1.8.1.4

--
Gleb.