linux-kernel - Re: [PATCH v3 7/8] KVM: Optimize gfn lookup in kvm_zap_gfn

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Date:   Wed, 26 May 2021 17:33:13 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 7/8] KVM: Optimize gfn lookup in kvm_zap_gfn_range()

On Sun, May 16, 2021, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
> 
> 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@...cle.com>
> ---
>  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.

> +			memslot = container_of(node, struct kvm_memory_slot,
> +					       gfn_node[idxactive]);
> +
> +			/*
> +			 * If this slot starts beyond or at the end of the range so does
> +			 * every next one
> +			 */
> +			if (memslot->base_gfn >= gfn_start + gfn_end)
> +				break;
> +
>  			start = max(gfn_start, memslot->base_gfn);
>  			end = min(gfn_end, memslot->base_gfn + memslot->npages);
>  			if (start >= end)