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));
