From: Peter Xu <peterx@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Maxim Levitsky <mlevitsk@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v2 9/9] KVM: X86: Optimize zapping rmap
Date: Wed, 28 Jul 2021 18:01:05 -0400 [thread overview]
Message-ID: <YQHTocEdMzsJQuzL@t490s> (raw)
In-Reply-To: <YQHOdhMoFW821HAu@google.com>
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
next prev parent reply other threads:[~2021-07-28 22:01 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-25 15:32 [PATCH v2 0/9] KVM: X86: Some light optimizations on rmap logic Peter Xu
2021-06-25 15:32 ` [PATCH v2 1/9] KVM: X86: Add per-vm stat for max rmap list size Peter Xu
2021-06-25 15:32 ` [PATCH v2 2/9] KVM: Introduce kvm_get_kvm_safe() Peter Xu
2021-07-26 13:42 ` Paolo Bonzini
2021-06-25 15:32 ` [PATCH v2 3/9] KVM: Allow to have arch-specific per-vm debugfs files Peter Xu
2021-06-25 15:32 ` [PATCH v2 4/9] KVM: X86: Introduce pte_list_count() helper Peter Xu
2021-06-25 15:32 ` [PATCH v2 5/9] KVM: X86: Introduce kvm_mmu_slot_lpages() helpers Peter Xu
2021-06-25 15:32 ` [PATCH v2 6/9] KVM: X86: Introduce mmu_rmaps_stat per-vm debugfs file Peter Xu
2021-06-25 15:34 ` [PATCH v2 7/9] KVM: X86: MMU: Tune PTE_LIST_EXT to be bigger Peter Xu
2021-07-28 21:01 ` Sean Christopherson
2021-06-25 15:34 ` [PATCH v2 8/9] KVM: X86: Optimize pte_list_desc with per-array counter Peter Xu
2021-07-28 21:04 ` Sean Christopherson
2021-07-28 21:51 ` Peter Xu
2021-07-29 9:33 ` Paolo Bonzini
2021-07-29 15:53 ` Peter Xu
2021-07-30 15:45 ` Peter Xu
2021-06-25 15:34 ` [PATCH v2 9/9] KVM: X86: Optimize zapping rmap Peter Xu
2021-07-28 21:39 ` Sean Christopherson
2021-07-28 22:01 ` Peter Xu [this message]
2021-07-28 22:31 ` Sean Christopherson
2021-07-29 9:35 ` Paolo Bonzini
2021-07-26 13:05 ` [PATCH v2 0/9] KVM: X86: Some light optimizations on rmap logic Paolo Bonzini
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