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SubjectRe: [PATCH v2 9/9] KVM: X86: Optimize zapping rmap
On Wed, Jul 28, 2021 at 09:39:02PM +0000, Sean Christopherson wrote:
> On Fri, Jun 25, 2021, Peter Xu wrote:
> > Using rmap_get_first() and rmap_remove() for zapping a huge rmap list could be
> > slow. The easy way is to travers the rmap list, collecting the a/d bits and
> > free the slots along the way.
> >
> > Provide a pte_list_destroy() and do exactly that.
> >
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> > arch/x86/kvm/mmu/mmu.c | 45 +++++++++++++++++++++++++++++++-----------
> > 1 file changed, 33 insertions(+), 12 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index ba0258bdebc4..45aac78dcabc 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -1014,6 +1014,38 @@ unsigned int pte_list_count(struct kvm_rmap_head *rmap_head)
> > return count;
> > }
> >
> > +/* Return true if rmap existed and callback called, false otherwise */
> > +static bool pte_list_destroy(struct kvm_rmap_head *rmap_head,
> > + int (*callback)(u64 *sptep))
> > +{
> > + struct pte_list_desc *desc, *next;
> > + int i;
> > +
> > + if (!rmap_head->val)
> > + return false;
> > +
> > + if (!(rmap_head->val & 1)) {
> > + if (callback)
> > + callback((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]);
> > + next = desc->more;
> > + mmu_free_pte_list_desc(desc);
> > + desc = next;
>
> Alternatively,
>
> desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
> for ( ; desc; desc = next) {
> for (i = 0; i < desc->spte_count; i++)
> mmu_spte_clear_track_bits((u64 *)rmap_head->val);
> next = desc->more;
> mmu_free_pte_list_desc(desc);
> }
>
> > + }
> > +out:
> > + /* rmap_head is meaningless now, remember to reset it */
> > + rmap_head->val = 0;
> > + return true;
>
> 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.
If that'll be a performance concern, no objection to hard code it.

>
> E.g. why not this?
>
> static bool kvm_zap_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
> const struct kvm_memory_slot *slot)
> {
> struct pte_list_desc *desc, *next;
> int i;
>
> if (!rmap_head->val)
> return false;
>
> if (!(rmap_head->val & 1)) {
> mmu_spte_clear_track_bits((u64 *)rmap_head->val);
> goto out;
> }
>
> desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
> 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);
> }
> out:
> /* rmap_head is meaningless now, remember to reset it */
> rmap_head->val = 0;
> return true;
> }

Looks good, but so far I've no strong opinion on this. I'll leave it for Paolo
to decide.

Thanks!

--
Peter Xu
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