On Wed, Mar 02, 2022, Paolo Bonzini wrote: > On 2/26/22 01:15, Sean Christopherson wrote: > > Zap defunct roots, a.k.a. roots that have been invalidated after their > > last reference was initially dropped, asynchronously via the system work > > queue instead of forcing the work upon the unfortunate task that happened > > to drop the last reference. > > > > If a vCPU task drops the last reference, the vCPU is effectively blocked > > by the host for the entire duration of the zap. If the root being zapped > > happens be fully populated with 4kb leaf SPTEs, e.g. due to dirty logging > > being active, the zap can take several hundred seconds. Unsurprisingly, > > most guests are unhappy if a vCPU disappears for hundreds of seconds. > > > > E.g. running a synthetic selftest that triggers a vCPU root zap with > > ~64tb of guest memory and 4kb SPTEs blocks the vCPU for 900+ seconds. > > Offloading the zap to a worker drops the block time to <100ms. > > > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> > > --- > > Do we even need kvm_tdp_mmu_zap_invalidated_roots() now? That is, > something like the following: Nice! I initially did something similar (moving invalidated roots to a separate list), but never circled back to idea after implementing the worker stuff. > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index bd3625a875ef..5fd8bc858c6f 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -5698,6 +5698,16 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm) > { > lockdep_assert_held(&kvm->slots_lock); > + /* > + * kvm_tdp_mmu_invalidate_all_roots() needs a nonzero reference > + * count. If we're dying, zap everything as it's going to happen > + * soon anyway. > + */ > + if (!refcount_read(&kvm->users_count)) { > + kvm_mmu_zap_all(kvm); > + return; > + } I'd prefer we make this an assertion and shove this logic to set_nx_huge_pages(), because in that case there's no need to zap anything, the guest can never run again. E.g. (I'm trying to remember why I didn't do this before...) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index b2c1c4eb6007..d4d25ab88ae7 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -6132,7 +6132,8 @@ static int set_nx_huge_pages(const char *val, const struct kernel_param *kp) list_for_each_entry(kvm, &vm_list, vm_list) { mutex_lock(&kvm->slots_lock); - kvm_mmu_zap_all_fast(kvm); + if (refcount_read(&kvm->users_count)) + kvm_mmu_zap_all_fast(kvm); mutex_unlock(&kvm->slots_lock); wake_up_process(kvm->arch.nx_lpage_recovery_thread); > + > write_lock(&kvm->mmu_lock); > trace_kvm_mmu_zap_all_fast(kvm); > @@ -5732,20 +5742,6 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm) > kvm_zap_obsolete_pages(kvm); > write_unlock(&kvm->mmu_lock); > - > - /* > - * Zap the invalidated TDP MMU roots, all SPTEs must be dropped before > - * returning to the caller, e.g. if the zap is in response to a memslot > - * deletion, mmu_notifier callbacks will be unable to reach the SPTEs > - * associated with the deleted memslot once the update completes, and > - * Deferring the zap until the final reference to the root is put would > - * lead to use-after-free. > - */ > - if (is_tdp_mmu_enabled(kvm)) { > - read_lock(&kvm->mmu_lock); > - kvm_tdp_mmu_zap_invalidated_roots(kvm); > - read_unlock(&kvm->mmu_lock); > - } > } > static bool kvm_has_zapped_obsolete_pages(struct kvm *kvm) > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c > index cd1bf68e7511..af9db5b8f713 100644 > --- a/arch/x86/kvm/mmu/tdp_mmu.c > +++ b/arch/x86/kvm/mmu/tdp_mmu.c > @@ -142,10 +142,12 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root, > WARN_ON(!root->tdp_mmu_page); > /* > - * The root now has refcount=0 and is valid. Readers cannot acquire > - * a reference to it (they all visit valid roots only, except for > - * kvm_tdp_mmu_zap_invalidated_roots() which however does not acquire > - * any reference itself. > + * The root now has refcount=0. It is valid, but readers already > + * cannot acquire a reference to it because kvm_tdp_mmu_get_root() > + * rejects it. This remains true for the rest of the execution > + * of this function, because readers visit valid roots only One thing that keeps tripping me up is the "readers" verbiage. I get confused because taking mmu_lock for read vs. write doesn't really have anything to do with reading or writing state, e.g. "readers" still write SPTEs, and so I keep thinking "readers" means anything iterating over the set of roots. Not sure if there's a shorthand that won't be confusing. > + * (except for tdp_mmu_zap_root_work(), which however operates only > + * on one specific root and does not acquire any reference itself). > > * > * Even though there are flows that need to visit all roots for > * correctness, they all take mmu_lock for write, so they cannot yet ... > It passes a smoke test, and also resolves the debate on the fate of patch 1. +1000, I love this approach. Do you want me to work on a v3, or shall I let you have the honors?