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Date: Wed, 19 May 2021 23:07:03 +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 4/8] KVM: Introduce memslots hva tree
Nit: something like "KVM: Use interval tree to do fast hva lookup in memslots"
would be more helpful when perusing the shortlogs. Stating that a tree is being
added doesn't provide any hint as to why, or even the what is somewhat unclear.
On Sun, May 16, 2021, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
>
> The current memslots implementation only allows quick binary search by gfn,
> quick lookup by hva is not possible - the implementation has to do a linear
> scan of the whole memslots array, even though the operation being performed
> might apply just to a single memslot.
>
> This significantly hurts performance of per-hva operations with higher
> memslot counts.
>
> Since hva ranges can overlap between memslots an interval tree is needed
> for tracking them.
>
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
> ---
...
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index d3a35646dfd8..f59847b6e9b3 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -27,6 +27,7 @@
> #include <linux/rcuwait.h>
> #include <linux/refcount.h>
> #include <linux/nospec.h>
> +#include <linux/interval_tree.h>
> #include <linux/hashtable.h>
> #include <asm/signal.h>
>
> @@ -358,6 +359,7 @@ static inline int kvm_vcpu_exiting_guest_mode(struct kvm_vcpu *vcpu)
>
> struct kvm_memory_slot {
> struct hlist_node id_node;
> + struct interval_tree_node hva_node;
> gfn_t base_gfn;
> unsigned long npages;
> unsigned long *dirty_bitmap;
> @@ -459,6 +461,7 @@ static inline int kvm_arch_vcpu_memslots_id(struct kvm_vcpu *vcpu)
> */
> struct kvm_memslots {
> u64 generation;
> + struct rb_root_cached hva_tree;
> /* The mapping table from slot id to the index in memslots[]. */
> DECLARE_HASHTABLE(id_hash, 7);
> atomic_t lru_slot;
> @@ -679,6 +682,11 @@ static inline struct kvm_memslots *kvm_vcpu_memslots(struct kvm_vcpu *vcpu)
> return __kvm_memslots(vcpu->kvm, as_id);
> }
>
> +#define kvm_for_each_hva_range_memslot(node, slots, start, last) \
kvm_for_each_memslot_in_range()? Or kvm_for_each_memslot_in_hva_range()?
Please add a comment about whether start is inclusive or exclusive.
I'd also be in favor of hiding this in kvm_main.c, just above the MMU notifier
usage. It'd be nice to discourage arch code from adding lookups that more than
likely belong in generic code.
> + for (node = interval_tree_iter_first(&slots->hva_tree, start, last); \
> + node; \
> + node = interval_tree_iter_next(node, start, last)) \
> +
> static inline
> struct kvm_memory_slot *id_to_memslot(struct kvm_memslots *slots, int id)
> {
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 50f9bc9bb1e0..a55309432c9a 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -488,6 +488,9 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
> struct kvm_memslots *slots;
> int i, idx;
>
> + if (range->end == range->start || WARN_ON(range->end < range->staart))
I'm pretty sure both of these are WARNable offenses, i.e. they can be combined.
It'd also be a good idea to use WARN_ON_ONCE(); if a caller does manage to
trigger this, odds are good it will get spammed.
Also, does interval_tree_iter_first() explode if given bad inputs? If not, I'd
probably say just omit this entirely. If it does explode, it might be a good idea
to work the sanity check into the macro, even if the macro is hidden here.
> + return 0;
> +
> /* A null handler is allowed if and only if on_lock() is provided. */
> if (WARN_ON_ONCE(IS_KVM_NULL_FN(range->on_lock) &&
> IS_KVM_NULL_FN(range->handler)))
> @@ -507,15 +510,18 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
> }
>
> for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> + struct interval_tree_node *node;
> +
> slots = __kvm_memslots(kvm, i);
> - kvm_for_each_memslot(slot, slots) {
> + kvm_for_each_hva_range_memslot(node, slots,
> + range->start, range->end - 1) {
> unsigned long hva_start, hva_end;
>
> + slot = container_of(node, struct kvm_memory_slot,
> + hva_node);
Eh, let that poke out. The 80 limit is more of a guideline.
> hva_start = max(range->start, slot->userspace_addr);
> hva_end = min(range->end, slot->userspace_addr +
> (slot->npages << PAGE_SHIFT));
> - if (hva_start >= hva_end)
> - continue;
>
> /*
> * To optimize for the likely case where the address
> @@ -787,6 +793,7 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
> if (!slots)
> return NULL;
>
> + slots->hva_tree = RB_ROOT_CACHED;
> hash_init(slots->id_hash);
>
> return slots;
> @@ -1113,10 +1120,14 @@ static inline void kvm_memslot_delete(struct kvm_memslots *slots,
> atomic_set(&slots->lru_slot, 0);
>
> for (i = dmemslot - mslots; i < slots->used_slots; i++) {
> + interval_tree_remove(&mslots[i].hva_node, &slots->hva_tree);
> hash_del(&mslots[i].id_node);
I think it would make sense to add helpers for these? Not sure I like the names,
but it would certainly dedup the code a bit.
static void kvm_memslot_remove(struct kvm_memslots *slots,
struct kvm_memslot *memslot)
{
interval_tree_remove(&memslot->hva_node, &slots->hva_tree);
hash_del(&memslot->id_node);
}
static void kvm_memslot_insert(struct kvm_memslots *slots,
struct kvm_memslot *memslot)
{
interval_tree_insert(&memslot->hva_node, &slots->hva_tree);
hash_add(slots->id_hash, &memslot->id_node, memslot->id);
}
> +
> mslots[i] = mslots[i + 1];
> + interval_tree_insert(&mslots[i].hva_node, &slots->hva_tree);
> hash_add(slots->id_hash, &mslots[i].id_node, mslots[i].id);
> }
> + interval_tree_remove(&mslots[i].hva_node, &slots->hva_tree);
> hash_del(&mslots[i].id_node);
> mslots[i] = *memslot;
> }
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