linux-kernel - Re: [PATCH v2 9/9] KVM: X86: Optimize zapping rmap

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Date:   Wed, 28 Jul 2021 22:31:51 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 9/9] KVM: X86: Optimize zapping rmap

On Wed, Jul 28, 2021, Peter Xu wrote:
> On Wed, Jul 28, 2021 at 09:39:02PM +0000, Sean Christopherson wrote:
> > On Fri, Jun 25, 2021, Peter Xu wrote:
> > Why implement this as a generic method with a callback?  gcc is suprisingly
> > astute in optimizing callback(), but I don't see the point of adding a complex
> > helper that has a single caller, and is extremely unlikely to gain new callers.
> > Or is there another "zap everything" case I'm missing?
> 
> No other case; it's just that pte_list_*() helpers will be more self-contained.

Eh, but this flow is as much about rmaps as it is about pte_list.

> If that'll be a performance concern, no objection to hard code it.

It's more about unnecessary complexity than it is about performance, e.g. gcc-10
generates identical code for both version (which did surprise the heck out of me).

If we really want to isolate pte_list_destroy(), I would vote for something like
this (squashed in).   pte_list_remove() already calls mmu_spte_clear_track_bits(),
so that particular separation of concerns has already gone out the window.

 
-/* Return true if rmap existed and callback called, false otherwise */
-static bool pte_list_destroy(struct kvm_rmap_head *rmap_head,
-                            void (*callback)(u64 *sptep))
+static bool pte_list_destroy(struct kvm_rmap_head *rmap_head)
 {
        struct pte_list_desc *desc, *next;
        int i;
@@ -1013,20 +1011,16 @@ static bool pte_list_destroy(struct kvm_rmap_head *rmap_head,
                return false;
 
        if (!(rmap_head->val & 1)) {
-               if (callback)
-                       callback((u64 *)rmap_head->val);
+               mmu_spte_clear_track_bits((u64 *)rmap_head->val);
                goto out;
        }
 
        desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
-
-       while (desc) {
-               if (callback)
-                       for (i = 0; i < desc->spte_count; i++)
-                               callback(desc->sptes[i]);
+       for ( ; desc; desc = next) {
+               for (i = 0; i < desc->spte_count; i++)
+                       mmu_spte_clear_track_bits(desc->sptes[i]);
                next = desc->more;
                mmu_free_pte_list_desc(desc);
-               desc = next;
        }
 out:
        /* rmap_head is meaningless now, remember to reset it */
@@ -1422,22 +1416,17 @@ static bool rmap_write_protect(struct kvm_vcpu *vcpu, u64 gfn)
        return kvm_mmu_slot_gfn_write_protect(vcpu->kvm, slot, gfn, PG_LEVEL_4K);
 }
 
-static void mmu_spte_clear_track_bits_cb(u64 *sptep)
-{
-       mmu_spte_clear_track_bits(sptep);
-}
-
 static bool kvm_zap_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
                          const struct kvm_memory_slot *slot)
 {
-       return pte_list_destroy(rmap_head, mmu_spte_clear_track_bits_cb);
+       return pte_list_destroy(rmap_head);
 }
 
 static bool kvm_unmap_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
                            struct kvm_memory_slot *slot, gfn_t gfn, int level,
                            pte_t unused)
 {
-       return kvm_zap_rmapp(kvm, rmap_head, slot);
+       return pte_list_destroy(rmap_head);
 }
 
 static bool kvm_set_pte_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,