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: <CADrL8HV3nNaKXkhX5zC2hN5+44AOZtoCvL83_x8kbkTVB3rHhg@mail.gmail.com>
Date: Mon, 27 Jan 2025 11:57:17 -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 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.