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

[Gleb Natapov]

On Tue, Oct 05, 2010 at 01:57:38PM -0300, Marcelo Tosatti wrote:
> 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).
> 
I do not understand what do you mean here. kvm_memslots structure itself
is not cached only various translation that use it are cached. Translation
result are never used if kvm_memslots was changed.

> Also should zap any cached entries on overflow, otherwise malicious
> userspace could make use of stale slots:
> 
There is only one cached entry at each given time. User who wants to
write into guest memory often defines gfn_to_hva_cache variable
somewhere. Init it with kvm_gfn_to_hva_cache_init() and then calls
kvm_write_guest_cached() on it. If there was no slot changes in between
cached translation are used. Otherwise cache is recalculated.

> > +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

--
			Gleb.
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