linux-kernel - Re: [PATCH v2 25/28] KVM: x86/mmu: Allow zapping collapsible SPTEs to use MMU read lock

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Message-ID: <CANgfPd_t1umcmiFzaUwsUwAAvOePbdxn5nY5y9NcoROYv5HEWg@mail.gmail.com>
Date:   Wed, 3 Feb 2021 10:51:49 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH v2 25/28] KVM: x86/mmu: Allow zapping collapsible SPTEs to
 use MMU read lock

On Wed, Feb 3, 2021 at 3:34 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 02/02/21 19:57, Ben Gardon wrote:
> > @@ -1485,7 +1489,9 @@ void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
> >       struct kvm_mmu_page *root;
> >       int root_as_id;
> >
> > -     for_each_tdp_mmu_root_yield_safe(kvm, root, false) {
> > +     read_lock(&kvm->mmu_lock);
> > +
> > +     for_each_tdp_mmu_root_yield_safe(kvm, root, true) {
> >               root_as_id = kvm_mmu_page_as_id(root);
> >               if (root_as_id != slot->as_id)
> >                       continue;
> > @@ -1493,6 +1499,8 @@ void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
> >               zap_collapsible_spte_range(kvm, root, slot->base_gfn,
> >                                          slot->base_gfn + slot->npages);
> >       }
> > +
> > +     read_unlock(&kvm->mmu_lock);
> >  }
>
>
> I'd prefer the functions to be consistent about who takes the lock,
> either mmu.c or tdp_mmu.c.  Since everywhere else you're doing it in
> mmu.c, that would be:
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 0554d9c5c5d4..386ee4b703d9 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5567,10 +5567,13 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
>         write_lock(&kvm->mmu_lock);
>         slot_handle_leaf(kvm, (struct kvm_memory_slot *)memslot,
>                          kvm_mmu_zap_collapsible_spte, true);
> +       write_unlock(&kvm->mmu_lock);
>
> -       if (kvm->arch.tdp_mmu_enabled)
> +       if (kvm->arch.tdp_mmu_enabled) {
> +               read_lock(&kvm->mmu_lock);
>                 kvm_tdp_mmu_zap_collapsible_sptes(kvm, memslot);
> -       write_unlock(&kvm->mmu_lock);
> +               read_unlock(&kvm->mmu_lock);
> +       }
>   }
>
>   void kvm_arch_flush_remote_tlbs_memslot(struct kvm *kvm,
>
> and just lockdep_assert_held_read here.

That makes sense to me, I agree keeping it consistent is probably a good idea.

>
> > -             tdp_mmu_set_spte(kvm, &iter, 0);
> > -
> > -             spte_set = true;
>
> Is it correct to remove this assignment?

No, it was not correct to remove it. Thank you for catching that.

>
> Paolo
>