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

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Message-ID: <99a961d2-d46c-ead9-1138-45f6587a60ed@maciej.szmigiero.name>
Date:   Tue, 1 Jun 2021 22:25:19 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Sean Christopherson <seanjc@...gle.com>
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 26.05.2021 19:33, Sean Christopherson wrote:
> 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.

That's a good idea, will do.

Thanks,
Maciej