Re: [PATCH v3 7/8] KVM: Optimize gfn lookup in kvm_zap_gfn_range()

[Maciej S. Szmigiero]

On 26.05.2021 19:33, Sean Christopherson wrote:
> On Sun, May 16, 2021, Maciej S. Szmigiero wrote:
> > From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>
> >
> > Introduce a memslots gfn upper bound operation and use it to optimize
> > kvm_zap_gfn_range().
> > This way this handler can do a quick lookup for intersecting gfns and won't
> > have to do a linear scan of the whole memslot set.
> >
> > Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/mmu/mmu.c   | 41 ++++++++++++++++++++++++++++++++++++++--
> >   include/linux/kvm_host.h | 22 +++++++++++++++++++++
> >   2 files changed, 61 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 7222b552d139..f23398cf0316 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -5490,14 +5490,51 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
> >   	int i;
> >   	bool flush = false;
> >   
> > +	if (gfn_end == gfn_start || WARN_ON(gfn_end < gfn_start))
> > +		return;
> > +
> >   	write_lock(&kvm->mmu_lock);
> >   	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> > -		int ctr;
> > +		int idxactive;
> > +		struct rb_node *node;
> >   
> >   		slots = __kvm_memslots(kvm, i);
> > -		kvm_for_each_memslot(memslot, ctr, slots) {
> > +		idxactive = kvm_memslots_idx(slots);
> > +
> > +		/*
> > +		 * Find the slot with the lowest gfn that can possibly intersect with
> > +		 * the range, so we'll ideally have slot start <= range start
> > +		 */
> > +		node = kvm_memslots_gfn_upper_bound(slots, gfn_start);
> > +		if (node) {
> > +			struct rb_node *pnode;
> > +
> > +			/*
> > +			 * A NULL previous node means that the very first slot
> > +			 * already has a higher start gfn.
> > +			 * In this case slot start > range start.
> > +			 */
> > +			pnode = rb_prev(node);
> > +			if (pnode)
> > +				node = pnode;
> > +		} else {
> > +			/* a NULL node below means no slots */
> > +			node = rb_last(&slots->gfn_tree);
> > +		}
> > +
> > +		for ( ; node; node = rb_next(node)) {
> >   			gfn_t start, end;
>
> Can this be abstracted into something like:
>
> 		kvm_for_each_memslot_in_gfn_range(...) {
>
> 		}
>
> and share that implementation with kvm_check_memslot_overlap() in the next patch?
>
> I really don't think arch code should be poking into gfn_tree, and ideally arch
> code wouldn't even be aware that gfn_tree exists.

That's a good idea, will do.

Thanks,
Maciej