Re: [PATCH 09/28] KVM: x86/mmu: Require mmu_lock be held for write in unyielding root iter

[Ben Gardon]

On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> Assert that mmu_lock is held for write by users of the yield-unfriendly
> TDP iterator.  The nature of a shared walk means that the caller needs to
> play nice with other tasks modifying the page tables, which is more or
> less the same thing as playing nice with yielding.  Theoretically, KVM
> could gain a flow where it could legitimately take mmu_lock for read in
> a non-preemptible context, but that's highly unlikely and any such case
> should be viewed with a fair amount of scrutiny.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 12a28afce73f..3086c6dc74fb 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -159,11 +159,17 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
>                 if (kvm_mmu_page_as_id(_root) != _as_id) {              \
>                 } else
>
> -#define for_each_tdp_mmu_root(_kvm, _root, _as_id)                             \
> -       list_for_each_entry_rcu(_root, &_kvm->arch.tdp_mmu_roots, link,         \
> -                               lockdep_is_held_type(&kvm->mmu_lock, 0) ||      \
> -                               lockdep_is_held(&kvm->arch.tdp_mmu_pages_lock)) \
> +/*
> + * Iterate over all valid TDP MMU roots.  Requires that mmu_lock be held for
> + * write, the implication being that any flow that holds mmu_lock for read is
> + * inherently yield-friendly and should use the yielf-safe variant above.

Nit: *yield-safe

> + * Holding mmu_lock for write obviates the need for RCU protection as the list
> + * is guaranteed to be stable.
> + */
> +#define for_each_tdp_mmu_root(_kvm, _root, _as_id)                     \
> +       list_for_each_entry(_root, &_kvm->arch.tdp_mmu_roots, link)     \
>                 if (kvm_mmu_page_as_id(_root) != _as_id) {              \
> +                       lockdep_assert_held_write(&(_kvm)->mmu_lock);   \

Did you mean for this lockdep to only be hit in this uncommon
non-matching ASID case?

>                 } else
>
>  static union kvm_mmu_page_role page_role_for_level(struct kvm_vcpu *vcpu,
> @@ -1063,6 +1069,8 @@ static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
>         struct tdp_iter iter;
>         bool ret = false;
>
> +       lockdep_assert_held_write(&kvm->mmu_lock);
> +
>         rcu_read_lock();
>
>         /*
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>