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Message-ID: <20200508201355.GS27052@linux.intel.com>
Date: Fri, 8 May 2020 13:13:55 -0700
From: Sean Christopherson <sean.j.christopherson@...el.com>
To: Jon Cargille <jcargill@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Peter Feiner <pfeiner@...gle.com>
Subject: Re: [PATCH] kvm: x86 mmu: avoid mmu_page_hash lookup for
direct_map-only VM
On Fri, May 08, 2020 at 11:24:25AM -0700, Jon Cargille wrote:
> From: Peter Feiner <pfeiner@...gle.com>
>
> Optimization for avoiding lookups in mmu_page_hash. When there's a
> single direct root, a shadow page has at most one parent SPTE
> (non-root SPs have exactly one; the root has none). Thus, if an SPTE
> is non-present, it can be linked to a newly allocated SP without
> first checking if the SP already exists.
Some mechanical comments below. I'll think through the actual logic next
week, my brain needs to be primed anytime the MMU is involved :-)
> This optimization has proven significant in batch large SP shattering
> where the hash lookup accounted for 95% of the overhead.
>
> Signed-off-by: Peter Feiner <pfeiner@...gle.com>
> Signed-off-by: Jon Cargille <jcargill@...gle.com>
> Reviewed-by: Jim Mattson <jmattson@...gle.com>
>
> ---
> arch/x86/include/asm/kvm_host.h | 13 ++++++++
> arch/x86/kvm/mmu/mmu.c | 55 +++++++++++++++++++--------------
> 2 files changed, 45 insertions(+), 23 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index a239a297be33..9b70d764b626 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -913,6 +913,19 @@ struct kvm_arch {
> struct kvm_page_track_notifier_node mmu_sp_tracker;
> struct kvm_page_track_notifier_head track_notifier_head;
>
> + /*
> + * Optimization for avoiding lookups in mmu_page_hash. When there's a
> + * single direct root, a shadow page has at most one parent SPTE
> + * (non-root SPs have exactly one; the root has none). Thus, if an SPTE
> + * is non-present, it can be linked to a newly allocated SP without
> + * first checking if the SP already exists.
> + *
> + * False initially because there are no indirect roots.
> + *
> + * Guarded by mmu_lock.
> + */
> + bool shadow_page_may_have_multiple_parents;
Why make this a one-way bool? Wouldn't it be better to let this transition
back to '0' once all nested guests go away?
And maybe a shorter name that reflects what it tracks instead of how its
used, e.g. has_indirect_mmu or indirect_mmu_count.
> +
> struct list_head assigned_dev_head;
> struct iommu_domain *iommu_domain;
> bool iommu_noncoherent;
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index e618472c572b..d94552b0ed77 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2499,35 +2499,40 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> quadrant &= (1 << ((PT32_PT_BITS - PT64_PT_BITS) * level)) - 1;
> role.quadrant = quadrant;
> }
> - for_each_valid_sp(vcpu->kvm, sp, gfn) {
> - if (sp->gfn != gfn) {
> - collisions++;
> - continue;
> - }
>
> - if (!need_sync && sp->unsync)
> - need_sync = true;
> + if (vcpu->kvm->arch.shadow_page_may_have_multiple_parents ||
> + level == vcpu->arch.mmu->root_level) {
Might be worth a goto to preserve the for-loop.
> + for_each_valid_sp(vcpu->kvm, sp, gfn) {
> + if (sp->gfn != gfn) {
> + collisions++;
> + continue;
> + }
>
> - if (sp->role.word != role.word)
> - continue;
> + if (!need_sync && sp->unsync)
> + need_sync = true;
>
> - if (sp->unsync) {
> - /* The page is good, but __kvm_sync_page might still end
> - * up zapping it. If so, break in order to rebuild it.
> - */
> - if (!__kvm_sync_page(vcpu, sp, &invalid_list))
> - break;
> + if (sp->role.word != role.word)
> + continue;
>
> - WARN_ON(!list_empty(&invalid_list));
> - kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
> - }
> + if (sp->unsync) {
> + /* The page is good, but __kvm_sync_page might
> + * still end up zapping it. If so, break in
> + * order to rebuild it.
> + */
> + if (!__kvm_sync_page(vcpu, sp, &invalid_list))
> + break;
>
> - if (sp->unsync_children)
> - kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
> + WARN_ON(!list_empty(&invalid_list));
> + kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
> + }
>
> - __clear_sp_write_flooding_count(sp);
> - trace_kvm_mmu_get_page(sp, false);
> - goto out;
> + if (sp->unsync_children)
> + kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
> +
> + __clear_sp_write_flooding_count(sp);
> + trace_kvm_mmu_get_page(sp, false);
> + goto out;
> + }
> }
>
> ++vcpu->kvm->stat.mmu_cache_miss;
> @@ -3735,6 +3740,10 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
> gfn_t root_gfn, root_pgd;
> int i;
>
> + spin_lock(&vcpu->kvm->mmu_lock);
> + vcpu->kvm->arch.shadow_page_may_have_multiple_parents = true;
> + spin_unlock(&vcpu->kvm->mmu_lock);
Taking the lock every time is unnecessary, even if this is changed to a
refcount type variable, e.g.
if (!has_indirect_mmu) {
lock_and_set
}
or
if (atomic_inc_return(&indirect_mmu_count) == 1)
lock_and_unlock;
> +
> root_pgd = vcpu->arch.mmu->get_guest_pgd(vcpu);
> root_gfn = root_pgd >> PAGE_SHIFT;
>
> --
> 2.26.2.303.gf8c07b1a785-goog
>
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