Re: [PATCH v6 04/12] Add memory slot versioning and use it toprovide fast guest write interface

[Marcelo Tosatti]

On Mon, Oct 04, 2010 at 05:56:26PM +0200, Gleb Natapov wrote:
> Keep track of memslots changes by keeping generation number in memslots
> structure. Provide kvm_write_guest_cached() function that skips
> gfn_to_hva() translation if memslots was not changed since previous
> invocation.
> 
> Signed-off-by: Gleb Natapov <gleb@xxxxxxxxxx>
> ---
>  include/linux/kvm_host.h  |    7 +++++
>  include/linux/kvm_types.h |    7 +++++
>  virt/kvm/kvm_main.c       |   57 +++++++++++++++++++++++++++++++++++++++++---
>  3 files changed, 67 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index a08614e..4dff9a1 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -199,6 +199,7 @@ struct kvm_irq_routing_table {};
>  
>  struct kvm_memslots {
>  	int nmemslots;
> +	u32 generation;
>  	struct kvm_memory_slot memslots[KVM_MEMORY_SLOTS +
>  					KVM_PRIVATE_MEM_SLOTS];
>  };
> @@ -352,12 +353,18 @@ int kvm_write_guest_page(struct kvm *kvm, gfn_t gfn, const void *data,
>  			 int offset, int len);
>  int kvm_write_guest(struct kvm *kvm, gpa_t gpa, const void *data,
>  		    unsigned long len);
> +int kvm_write_guest_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
> +			   void *data, unsigned long len);
> +int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
> +			      gpa_t gpa);
>  int kvm_clear_guest_page(struct kvm *kvm, gfn_t gfn, int offset, int len);
>  int kvm_clear_guest(struct kvm *kvm, gpa_t gpa, unsigned long len);
>  struct kvm_memory_slot *gfn_to_memslot(struct kvm *kvm, gfn_t gfn);
>  int kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn);
>  unsigned long kvm_host_page_size(struct kvm *kvm, gfn_t gfn);
>  void mark_page_dirty(struct kvm *kvm, gfn_t gfn);
> +void mark_page_dirty_in_slot(struct kvm *kvm, struct kvm_memory_slot *memslot,
> +			     gfn_t gfn);
>  
>  void kvm_vcpu_block(struct kvm_vcpu *vcpu);
>  void kvm_vcpu_on_spin(struct kvm_vcpu *vcpu);
> diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> index 7ac0d4e..ee6eb71 100644
> --- a/include/linux/kvm_types.h
> +++ b/include/linux/kvm_types.h
> @@ -67,4 +67,11 @@ struct kvm_lapic_irq {
>  	u32 dest_id;
>  };
>  
> +struct gfn_to_hva_cache {
> +	u32 generation;
> +	gpa_t gpa;
> +	unsigned long hva;
> +	struct kvm_memory_slot *memslot;
> +};
> +
>  #endif /* __KVM_TYPES_H__ */
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index db58a1b..45ef50c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -687,6 +687,7 @@ skip_lpage:
>  		memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
>  		if (mem->slot >= slots->nmemslots)
>  			slots->nmemslots = mem->slot + 1;
> +		slots->generation++;
>  		slots->memslots[mem->slot].flags |= KVM_MEMSLOT_INVALID;
>  
>  		old_memslots = kvm->memslots;
> @@ -723,6 +724,7 @@ skip_lpage:
>  	memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
>  	if (mem->slot >= slots->nmemslots)
>  		slots->nmemslots = mem->slot + 1;
> +	slots->generation++;
>  
>  	/* actual memory is freed via old in kvm_free_physmem_slot below */
>  	if (!npages) {
> @@ -1247,6 +1249,47 @@ int kvm_write_guest(struct kvm *kvm, gpa_t gpa, const void *data,
>  	return 0;
>  }
>  
> +int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
> +			      gpa_t gpa)
> +{
> +	struct kvm_memslots *slots = kvm_memslots(kvm);
> +	int offset = offset_in_page(gpa);
> +	gfn_t gfn = gpa >> PAGE_SHIFT;
> +
> +	ghc->gpa = gpa;
> +	ghc->generation = slots->generation;
> +	ghc->memslot = gfn_to_memslot(kvm, gfn);
> +	ghc->hva = gfn_to_hva(kvm, gfn);
> +	if (!kvm_is_error_hva(ghc->hva))
> +		ghc->hva += offset;
> +	else
> +		return -EFAULT;
> +
> +	return 0;
> +}

Should use a unique kvm_memslots structure for the cache entry, since it
can change in between (use gfn_to_hva_memslot, etc on "slots" pointer).

Also should zap any cached entries on overflow, otherwise malicious
userspace could make use of stale slots:

> +void mark_page_dirty(struct kvm *kvm, gfn_t gfn)
> +{
> +	struct kvm_memory_slot *memslot;
> +
> +	memslot = gfn_to_memslot(kvm, gfn);
> +	mark_page_dirty_in_slot(kvm, memslot, gfn);
> +}
> +
>  /*
>   * The vCPU has executed a HLT instruction with in-kernel mode enabled.
>   */
> -- 
> 1.7.1
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