linux-kernel - Re: [PATCH 08/13] KVM: x86/mmu: Protect the tdp_mmu

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Date:   Thu, 1 Apr 2021 09:48:37 -0700
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 08/13] KVM: x86/mmu: Protect the tdp_mmu_roots list with RCU

On Thu, Apr 1, 2021 at 2:37 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 31/03/21 23:08, Ben Gardon wrote:
> > Protect the contents of the TDP MMU roots list with RCU in preparation
> > for a future patch which will allow the iterator macro to be used under
> > the MMU lock in read mode.
> >
> > Signed-off-by: Ben Gardon<bgardon@...gle.com>
> > ---
> >   arch/x86/kvm/mmu/tdp_mmu.c | 64 +++++++++++++++++++++-----------------
> >   1 file changed, 36 insertions(+), 28 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > +     spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> > +     list_del_rcu(&root->link);
> > +     spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
>
>
> Please update the comment above tdp_mmu_pages_lock in
> arch/x86/include/asm/kvm_host.h as well.

Ah yes, thank you for catching that. Will do.

>
> >  /* Only safe under the MMU lock in write mode, without yielding. */
> >  #define for_each_tdp_mmu_root(_kvm, _root)                           \
> > -     list_for_each_entry(_root, &_kvm->arch.tdp_mmu_roots, link)
> > +     list_for_each_entry_rcu(_root, &_kvm->arch.tdp_mmu_roots, link, \
> > +                             lockdep_is_held_write(&kvm->mmu_lock))
>
> This should also add "... ||
> lockdep_is_help(&kvm->arch.tdp_mmu_pages_lock)", if only for
> documentation purposes.

Good idea. I hope we never have a function try to protect its loop
over the roots with that lock, but it would be correct.

>
> Paolo
>