Re: [PATCH 3/3] KVM: MMU: retry #PF for softmmu

[Gleb Natapov]

On Thu, Nov 04, 2010 at 06:36:36PM +0800, Xiao Guangrong wrote:
> Retry #PF for softmmu only when the current vcpu has the same
> root shadow page as the time when #PF occurs. it means they
> have same paging environment.
> 
Avi had an idea to allocate spte at the fault time, get reference
to it and populate it on completion instead of prefaulting. How hard
will it be?

> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |    6 ++++++
>  arch/x86/kvm/mmu.c              |   33 ++++++++++++++++++++++++++++++++-
>  arch/x86/kvm/x86.c              |   16 ++++++++++++++--
>  virt/kvm/async_pf.c             |    1 +
>  4 files changed, 53 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 7f20f2c..b99ef7d 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -192,6 +192,8 @@ struct kvm_mmu_page {
>  	struct list_head link;
>  	struct hlist_node hash_link;
>  
> +	struct kref apfs_counter;
> +
>  	/*
>  	 * The following two entries are used to key the shadow page in the
>  	 * hash table.
> @@ -600,6 +602,7 @@ struct kvm_x86_ops {
>  struct kvm_arch_async_pf {
>  	u32 token;
>  	gfn_t gfn;
> +	struct kvm_mmu_page *root_sp;
>  };
>  
>  extern struct kvm_x86_ops *kvm_x86_ops;
> @@ -697,6 +700,8 @@ void kvm_inject_nmi(struct kvm_vcpu *vcpu);
>  
>  int fx_init(struct kvm_vcpu *vcpu);
>  
> +struct kvm_mmu_page *get_vcpu_root_sp(struct kvm_vcpu *vcpu, gva_t gva);
> +void kvm_mmu_release_apf_sp(struct kvm_mmu_page *sp);
>  void kvm_mmu_flush_tlb(struct kvm_vcpu *vcpu);
>  void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
>  		       const u8 *new, int bytes,
> @@ -822,6 +827,7 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
>  void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu,
>  			       struct kvm_async_pf *work);
>  bool kvm_arch_can_inject_async_page_present(struct kvm_vcpu *vcpu);
> +void kvm_arch_clear_async_pf(struct kvm_async_pf *work);
>  extern bool kvm_find_async_pf_gfn(struct kvm_vcpu *vcpu, gfn_t gfn);
>  
>  #endif /* _ASM_X86_KVM_HOST_H */
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index f3fad4f..60cc9f9 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -993,6 +993,19 @@ static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, int nr)
>  	percpu_counter_add(&kvm_total_used_mmu_pages, nr);
>  }
>  
> +static void free_shadow_page(struct kref *kref)
> +{
> +	struct kvm_mmu_page *sp;
> +
> +	sp = container_of(kref, struct kvm_mmu_page, apfs_counter);
> +	kmem_cache_free(mmu_page_header_cache, sp);
> +}
> +
> +void kvm_mmu_release_apf_sp(struct kvm_mmu_page *sp)
> +{
> +	kref_put(&sp->apfs_counter, free_shadow_page);;
> +}
> +
>  static void kvm_mmu_free_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
>  	ASSERT(is_empty_shadow_page(sp->spt));
> @@ -1001,7 +1014,7 @@ static void kvm_mmu_free_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  	__free_page(virt_to_page(sp->spt));
>  	if (!sp->role.direct)
>  		__free_page(virt_to_page(sp->gfns));
> -	kmem_cache_free(mmu_page_header_cache, sp);
> +	kvm_mmu_release_apf_sp(sp);
>  	kvm_mod_used_mmu_pages(kvm, -1);
>  }
>  
> @@ -1026,6 +1039,7 @@ static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu,
>  	sp->multimapped = 0;
>  	sp->parent_pte = parent_pte;
>  	kvm_mod_used_mmu_pages(vcpu->kvm, +1);
> +	kref_init(&sp->apfs_counter);
>  	return sp;
>  }
>  
> @@ -2597,11 +2611,28 @@ static int nonpaging_page_fault(struct kvm_vcpu *vcpu, gva_t gva,
>  			     error_code & PFERR_WRITE_MASK, gfn);
>  }
>  
> +struct kvm_mmu_page *get_vcpu_root_sp(struct kvm_vcpu *vcpu, gva_t gva)
> +{
> +	struct kvm_shadow_walk_iterator iterator;
> +	bool ret;
> +
> +	shadow_walk_init(&iterator, vcpu, gva);
> +	ret = shadow_walk_okay(&iterator);
> +	WARN_ON(!ret);
> +
> +	return page_header(__pa(iterator.sptep));
> +}
> +
>  static int kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gva_t gva, gfn_t gfn)
>  {
>  	struct kvm_arch_async_pf arch;
> +
>  	arch.token = (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
>  	arch.gfn = gfn;
> +	if (!tdp_enabled) {
> +		arch.root_sp = get_vcpu_root_sp(vcpu, gva);
> +		kref_get(&arch.root_sp->apfs_counter);
> +	}
>  
>  	return kvm_setup_async_pf(vcpu, gva, gfn, &arch);
>  }
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index aacc5eb..72d672f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6174,14 +6174,17 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
>  {
>  	int r;
>  
> -	if (!tdp_enabled || is_error_page(work->page))
> +	if (is_error_page(work->page))
>  		return;
>  
>  	r = kvm_mmu_reload(vcpu);
> +
>  	if (unlikely(r))
>  		return;
>  
> -	vcpu->arch.mmu.page_fault(vcpu, work->gva, 0, true);
> +	if (tdp_enabled ||
> +	      get_vcpu_root_sp(vcpu, work->gva) == work->arch.root_sp)
> +		vcpu->arch.mmu.page_fault(vcpu, work->gva, 0, true);
>  }
>  
>  static inline u32 kvm_async_pf_hash_fn(gfn_t gfn)
> @@ -6269,10 +6272,19 @@ void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
>  	}
>  }
>  
> +void kvm_arch_clear_async_pf(struct kvm_async_pf *work)
> +{
> +	if (!tdp_enabled)
> +		kvm_mmu_release_apf_sp(work->arch.root_sp);
> +}
> +
>  void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
>  				 struct kvm_async_pf *work)
>  {
>  	trace_kvm_async_pf_ready(work->arch.token, work->gva);
> +
> +	kvm_arch_clear_async_pf(work);
> +
>  	if (is_error_page(work->page))
>  		work->arch.token = ~0; /* broadcast wakeup */
>  	else
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index 74268b4..c3d4788 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -101,6 +101,7 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
>  				   typeof(*work), queue);
>  		cancel_work_sync(&work->work);
>  		list_del(&work->queue);
> +		kvm_arch_clear_async_pf(work);
>  		if (!work->done) /* work was canceled */
>  			kmem_cache_free(async_pf_cache, work);
>  	}
> -- 
> 1.7.0.4

--
			Gleb.
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