Re: [PATCH 08/13] KVM: x86/mmu: Protect the tdp_mmu_roots list with RCU

From: Ben Gardon <bgardon@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	Peter Xu <peterx@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Peter Shier <pshier@google.com>,
	Peter Feiner <pfeiner@google.com>,
	Junaid Shahid <junaids@google.com>,
	Jim Mattson <jmattson@google.com>,
	Yulei Zhang <yulei.kernel@gmail.com>,
	Wanpeng Li <kernellwp@gmail.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Subject: Re: [PATCH 08/13] KVM: x86/mmu: Protect the tdp_mmu_roots list with RCU
Date: Thu, 1 Apr 2021 09:48:37 -0700	[thread overview]
Message-ID: <CANgfPd87mz5Gqod6iRsqbdbUKxhETTrMp1R6ZqSerjvB0+mO8w@mail.gmail.com> (raw)
In-Reply-To: <a030f6a6-4092-7c70-af4e-148debb801cc@redhat.com>

On Thu, Apr 1, 2021 at 2:37 AM Paolo Bonzini <pbonzini@redhat.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@google.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
>