Re: [PATCH 10/15] KVM: MMU: lockless walking shadow page table

Marcelo Tosatti
2011-06-20T17:27:09Z

On Tue, Jun 07, 2011 at 09:04:34PM +0800, Xiao Guangrong wrote:
> Using rcu to protect shadow pages table to be freed, so we can safely walk it,
> it should run fast and is needed by mmio page fault
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
> ---
> arch/x86/include/asm/kvm_host.h | 4 ++
> arch/x86/kvm/mmu.c | 79 ++++++++++++++++++++++++++++++---------
> arch/x86/kvm/mmu.h | 4 +-
> arch/x86/kvm/vmx.c | 2 +-
> 4 files changed, 69 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 326af42..260582b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -232,6 +232,8 @@ struct kvm_mmu_page {
> unsigned int unsync_children;
> unsigned long parent_ptes; /* Reverse mapping for parent_pte */
> DECLARE_BITMAP(unsync_child_bitmap, 512);
> +
> + struct rcu_head rcu;
> };
>
> struct kvm_pv_mmu_op_buffer {
> @@ -478,6 +480,8 @@ struct kvm_arch {
> u64 hv_guest_os_id;
> u64 hv_hypercall;
>
> + atomic_t reader_counter;
> +
> #ifdef CONFIG_KVM_MMU_AUDIT
> int audit_point;
> #endif
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 9f3a746..52d4682 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1675,6 +1675,30 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> return ret;
> }
>
> +static void free_mmu_pages_unlock_parts(struct list_head *invalid_list)
> +{
> + struct kvm_mmu_page *sp;
> +
> + list_for_each_entry(sp, invalid_list, link)
> + kvm_mmu_free_lock_parts(sp);
> +}
> +
> +static void free_invalid_pages_rcu(struct rcu_head *head)
> +{
> + struct kvm_mmu_page *next, *sp;
> +
> + sp = container_of(head, struct kvm_mmu_page, rcu);
> + while (sp) {
> + if (!list_empty(&sp->link))
> + next = list_first_entry(&sp->link,
> + struct kvm_mmu_page, link);
> + else
> + next = NULL;
> + kvm_mmu_free_unlock_parts(sp);
> + sp = next;
> + }
> +}
> +
> static void kvm_mmu_commit_zap_page(struct kvm *kvm,
> struct list_head *invalid_list)
> {
> @@ -1685,6 +1709,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>
> kvm_flush_remote_tlbs(kvm);
>
> + if (atomic_read(&kvm->arch.reader_counter)) {
> + free_mmu_pages_unlock_parts(invalid_list);
> + sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
> + list_del_init(invalid_list);
> + call_rcu(&sp->rcu, free_invalid_pages_rcu);
> + return;
> + }

This is probably wrong, the caller wants the page to be zapped by the
time the function returns, not scheduled sometime in the future.

> +
> do {
> sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
> WARN_ON(!sp->role.invalid || sp->root_count);
> @@ -2601,6 +2633,35 @@ static gpa_t nonpaging_gva_to_gpa_nested(struct kvm_vcpu *vcpu, gva_t vaddr,
> return vcpu->arch.nested_mmu.translate_gpa(vcpu, vaddr, access);
> }
>
> +int kvm_mmu_walk_shadow_page_lockless(struct kvm_vcpu *vcpu, u64 addr,
> + u64 sptes[4])
> +{
> + struct kvm_shadow_walk_iterator iterator;
> + int nr_sptes = 0;
> +
> + rcu_read_lock();
> +
> + atomic_inc(&vcpu->kvm->arch.reader_counter);
> + /* Increase the counter before walking shadow page table */
> + smp_mb__after_atomic_inc();
> +
> + for_each_shadow_entry(vcpu, addr, iterator) {
> + sptes[iterator.level-1] = *iterator.sptep;
> + nr_sptes++;
> + if (!is_shadow_present_pte(*iterator.sptep))
> + break;
> + }

Why is lockless access needed for the MMIO optimization? Note the spte
contents are copied to the array here are used for debugging purposes
only, their contents are potentially stale.