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

[Sean Christopherson <seanjc@google.com>]

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));