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SubjectRe: [PATCH v5 01/13] KVM: x86: Cache total page count to avoid traversing the memslot array
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On 20.10.2021 00:31, Sean Christopherson wrote:
> On Tue, Oct 19, 2021, Sean Christopherson wrote:
>> On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote:
>>> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
>>>
>>> There is no point in recalculating from scratch the total number of pages
>>> in all memslots each time a memslot is created or deleted.
>>>
>>> Just cache the value and update it accordingly on each such operation so
>>> the code doesn't need to traverse the whole memslot array each time.
>>>
>>> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
>>> ---
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 28ef14155726..65fdf27b9423 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -11609,9 +11609,23 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>>> const struct kvm_memory_slot *new,
>>> enum kvm_mr_change change)
>>> {
>>> - if (!kvm->arch.n_requested_mmu_pages)
>>> - kvm_mmu_change_mmu_pages(kvm,
>>> - kvm_mmu_calculate_default_mmu_pages(kvm));
>>> + if (change == KVM_MR_CREATE)
>>> + kvm->arch.n_memslots_pages += new->npages;
>>> + else if (change == KVM_MR_DELETE) {
>>> + WARN_ON(kvm->arch.n_memslots_pages < old->npages);
>>> + kvm->arch.n_memslots_pages -= old->npages;
>>> + }
>>> +
>>> + if (!kvm->arch.n_requested_mmu_pages) {
>>
>> Hmm, once n_requested_mmu_pages is set it can't be unset. That means this can be
>> further optimized to skip avoid taking mmu_lock on flags-only changes (and
>> memslot movement). E.g.
>>
>> if (!kvm->arch.n_requested_mmu_pages &&
>> (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) {
>>
>> }
>>
>> It's a little risky, but kvm_vm_ioctl_set_nr_mmu_pages() would need to be modified
>> to allow clearing n_requested_mmu_pages and it already takes slots_lock, so IMO
>> it's ok to force kvm_vm_ioctl_set_nr_mmu_pages() to recalculate pages if it wants
>> to allow reverting back to the default.
>
> Doh, and then I read patch 2...
>
> I would swap the order of patch 2 and patch 1, that way the optimization patch is
> super simple, and you don't end up reworking a bunch of code that was added in the
> immediately preceding patch. E.g. as a first patch
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 28ef14155726..f3b1aed08566 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11609,7 +11609,8 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> const struct kvm_memory_slot *new,
> enum kvm_mr_change change)
> {
> - if (!kvm->arch.n_requested_mmu_pages)
> + if (!kvm->arch.n_requested_mmu_pages &&
> + (change == KVM_MR_CREATE || change == KVM_MR_DELETE))
> kvm_mmu_change_mmu_pages(kvm,
> kvm_mmu_calculate_default_mmu_pages(kvm));
>
>
>

Will do.

Thanks,
Maciej

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