Re: [PATCH 13/28] KVM: x86/mmu: Drop RCU after processing each root in MMU notifier hooks

[Ben Gardon]

On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> Drop RCU protection after processing each root when handling MMU notifier
> hooks that aren't the "unmap" path, i.e. aren't zapping.  Temporarily
> drop RCU to let RCU do its thing between roots, and to make it clear that
> there's no special behavior that relies on holding RCU across all roots.
>
> Currently, the RCU protection is completely superficial, it's necessary
> only to make rcu_dereference() of SPTE pointers happy.  A future patch
> will rely on holding RCU as a proxy for vCPUs in the guest, e.g. to
> ensure shadow pages aren't freed before all vCPUs do a TLB flush (or
> rather, acknowledge the need for a flush), but in that case RCU needs to
> be held until the flush is complete if and only if the flush is needed
> because a shadow page may have been removed.  And except for the "unmap"
> path, MMU notifier events cannot remove SPs (don't toggle PRESENT bit,
> and can't change the PFN for a SP).
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>

> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 06b500fab248..3ff7b4cd7d0e 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1079,18 +1079,19 @@ static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
>
>         lockdep_assert_held_write(&kvm->mmu_lock);
>
> -       rcu_read_lock();
> -
>         /*
>          * Don't support rescheduling, none of the MMU notifiers that funnel
>          * into this helper allow blocking; it'd be dead, wasteful code.
>          */
>         for_each_tdp_mmu_root(kvm, root, range->slot->as_id) {
> +               rcu_read_lock();
> +
>                 tdp_root_for_each_leaf_pte(iter, root, range->start, range->end)
>                         ret |= handler(kvm, &iter, range);
> +
> +               rcu_read_unlock();
>         }
>
> -       rcu_read_unlock();
>
>         return ret;
>  }
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>