Re: [PATCH v4 4/6] KVM: MMU: fast invalid all shadow pages

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, May 07, 2013 at 11:39:59AM +0800, Xiao Guangrong wrote:
> >>> Step 1) Fix kvm_mmu_zap_all's behaviour: introduce lockbreak via
> >>> spin_needbreak. Use generation numbers so that in case kvm_mmu_zap_all 
> >>> releases mmu_lock and reacquires it again, only shadow pages 
> >>> from the generation with which kvm_mmu_zap_all started are zapped (this
> >>> guarantees forward progress and eventual termination).
> >>>
> >>> kvm_mmu_zap_generation()
> >>> 	spin_lock(mmu_lock)
> >>> 	int generation = kvm->arch.mmu_generation;
> >>>
> >>> 	for_each_shadow_page(sp) {
> >>> 		if (sp->generation == kvm->arch.mmu_generation)
> >>> 			zap_page(sp)
> >>> 		if (spin_needbreak(mmu_lock)) {
> >>> 			kvm->arch.mmu_generation++;
> >>> 			cond_resched_lock(mmu_lock);
> >>> 		}
> >>> 	}
> >>>
> >>> kvm_mmu_zap_all()
> >>> 	spin_lock(mmu_lock)
> >>> 	for_each_shadow_page(sp) {
> >>> 		if (spin_needbreak(mmu_lock)) {
> >>> 			cond_resched_lock(mmu_lock);
> >>> 		}
> >>> 	}
> >>>
> >>> Use kvm_mmu_zap_generation for kvm_arch_flush_shadow_memslot.
> >>> Use kvm_mmu_zap_all for kvm_mmu_notifier_release,kvm_destroy_vm.
> >>>
> >>> This addresses the main problem: excessively long hold times 
> >>> of kvm_mmu_zap_all with very large guests.
> >>>
> >>> Do you see any problem with this logic? This was what i was thinking 
> >>> we agreed.
> >>
> >> No. I understand it and it can work.
> >>
> >> Actually, it is similar with Gleb's idea that "zapping stale shadow pages
> >> (and uses lock break technique)", after some discussion, we thought "only zap
> >> shadow pages that are reachable from the slot's rmap" is better, that is this
> >> patchset does.
> >> (https://lkml.org/lkml/2013/4/23/73)
> >>
> >>>
> >>> Step 2) Show that the optimization to zap only the roots is worthwhile
> >>> via benchmarking, and implement it.
> >>
> >> This is what i am confused. I can not understand how "zap only the roots"
> >> works. You mean these change?
> >>
> >> kvm_mmu_zap_generation()
> >>  	spin_lock(mmu_lock)
> >>  	int generation = kvm->arch.mmu_generation;
> >>
> >>  	for_each_shadow_page(sp) {
> >> 		/* Change here. */
> >> => 		if ((sp->generation == kvm->arch.mmu_generation) &&
> >> =>		      sp->root_count)
> >>  			zap_page(sp)
> >>
> >>  		if (spin_needbreak(mmu_lock)) {
> >>  			kvm->arch.mmu_generation++;
> >>  			cond_resched_lock(mmu_lock);
> >>  		}
> >>  	}
> >>
> >> If we do this, there will have shadow pages that are linked to invalid memslot's
> >> rmap. How do we handle these pages and the mmu-notify issue?

No, this is a full kvm_mmu_zap_page().

In step 2, after demonstrating and understanding kvm_mmu_zap_page()'s inefficiency (which
we are not certain about, given the four use cases of slot
deletion/move/create), use something smarter than plain
kvm_mmu_zap_page.

> >> Thanks!
> > 
> > By "zap only roots" i mean zapping roots plus generation number on
> > shadow pages. But this as a second step, after it has been demonstrated
> > its worthwhile.
> 
> Marcelo,
> 
> Sorry for my stupidity, still do not understand. Could you please show me the
> pseudocode and answer my questions above?

Hopefully its clear now?