linux-kernel - Re: [PATCH v8 04/11] KVM: x86/mmu: Relax locking for kvm_test_age_gfn and kvm_age

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Message-ID: <CADrL8HVczq2+3V6sQrNmGpFz_3-BsO5PZUm29Kg9LpvpoWnACw@mail.gmail.com>
Date: Mon, 27 Jan 2025 12:09:39 -0800
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: David Matlack <dmatlack@...gle.com>, David Rientjes <rientjes@...gle.com>, 
	Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, Wei Xu <weixugc@...gle.com>, 
	Yu Zhao <yuzhao@...gle.com>, Axel Rasmussen <axelrasmussen@...gle.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Venkatesh Srinivas <venkateshs@...gle.com>
Subject: Re: [PATCH v8 04/11] KVM: x86/mmu: Relax locking for kvm_test_age_gfn
 and kvm_age_gfn

On Mon, Jan 27, 2025 at 11:57 AM James Houghton <jthoughton@...gle.com> wrote:
>
> On Tue, Nov 5, 2024 at 10:43 AM James Houghton <jthoughton@...gle.com> wrote:
> >
> >  static inline u64 kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 old_spte,
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 4508d868f1cd..f5b4f1060fff 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -178,6 +178,15 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
> >                      ((_only_valid) && (_root)->role.invalid))) {               \
> >                 } else
> >
> > +/*
> > + * Iterate over all TDP MMU roots in an RCU read-side critical section.
> > + */
> > +#define for_each_valid_tdp_mmu_root_rcu(_kvm, _root, _as_id)                   \
> > +       list_for_each_entry_rcu(_root, &_kvm->arch.tdp_mmu_roots, link)         \
> > +               if ((_as_id >= 0 && kvm_mmu_page_as_id(_root) != _as_id) ||     \
> > +                   (_root)->role.invalid) {                                    \
> > +               } else
> > +
>
> Venkatesh noticed that this function is unused in this patch. This was
> a mistake in the latest rebase. The diff should have been applied:
>
> @@ -1192,15 +1206,15 @@ static bool __kvm_tdp_mmu_age_gfn_range(struct kvm *kvm,
>         struct tdp_iter iter;
>         bool ret = false;
>
> +       guard(rcu)();
> +
>         /*
>          * Don't support rescheduling, none of the MMU notifiers that funnel
>          * into this helper allow blocking; it'd be dead, wasteful code.  Note,
>          * this helper must NOT be used to unmap GFNs, as it processes only
>          * valid roots!
>          */
> -       for_each_valid_tdp_mmu_root(kvm, root, range->slot->as_id) {
> -               guard(rcu)();
> -
> +       for_each_valid_tdp_mmu_root_rcu(kvm, root, range->slot->as_id) {
>                 tdp_root_for_each_leaf_pte(iter, root, range->start,
> range->end) {
>                         if (!is_accessed_spte(iter.old_spte))
>                                 continue;
>
> This bug will show up as a LOCKDEP warning on CONFIG_PROVE_RCU_LIST=y
> kernels, as list_for_each_entry_rcu() is called outside of an RCU
> read-side critical section.

There I go lying again. The LOCKDEP warning that will show up is the
lockdep_assert_held_write(mmu_lock) in
kvm_lockdep_assert_mmu_lock_held().

The RCU lockdep WARN would hit if I had used
for_each_valid_tdp_mmu_root_rcu() without moving the RCU lock
acquisition.