On Sat, Feb 26, 2022, Sean Christopherson 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> Reviewed-by: Mingwei Zhang <mizhang@xxxxxxxxxx> > --- > arch/x86/kvm/mmu/tdp_mmu.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c > index 634a2838e117..4f460782a848 100644 > --- a/arch/x86/kvm/mmu/tdp_mmu.c > +++ b/arch/x86/kvm/mmu/tdp_mmu.c > @@ -1100,18 +1100,18 @@ static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm, > struct tdp_iter iter; > bool ret = false; > > - 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; > } > -- > 2.35.1.574.g5d30c73bfb-goog >