Re: [PATCH v5 01/13] KVM: x86: Cache total page count to avoid traversing the memslot array

[Maciej S. Szmigiero]

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@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > ---
> > > 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