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

[Ben Gardon]

On Thu, Apr 1, 2021 at 2:37 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >   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
>