On Thu, Mar 03, 2022, Paolo Bonzini wrote: > From: Sean Christopherson <seanjc@xxxxxxxxxx> > > Explicitly ignore the result of zap_gfn_range() when putting the last > reference to a TDP MMU root, and add a pile of comments to formalize the > TDP MMU's behavior of deferring TLB flushes to alloc/reuse. Note, this > only affects the !shared case, as zap_gfn_range() subtly never returns > true for "flush" as the flush is handled by tdp_mmu_zap_spte_atomic(). > > Putting the root without a flush is ok because even if there are stale > references to the root in the TLB, they are unreachable because KVM will > not run the guest with the same ASID without first flushing (where ASID > in this context refers to both SVM's explicit ASID and Intel's implicit > ASID that is constructed from VPID+PCID+EPT4A+etc...). > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> > Message-Id: <20220226001546.360188-5-seanjc@xxxxxxxxxx> > Reviewed-by: Mingwei Zhang <mizhang@xxxxxxxxxx> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > --- > arch/x86/kvm/mmu/mmu.c | 8 ++++++++ > arch/x86/kvm/mmu/tdp_mmu.c | 10 +++++++++- > 2 files changed, 17 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 32c041ed65cb..9a6df2d02777 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -5083,6 +5083,14 @@ int kvm_mmu_load(struct kvm_vcpu *vcpu) > kvm_mmu_sync_roots(vcpu); > > kvm_mmu_load_pgd(vcpu); > + > + /* > + * Flush any TLB entries for the new root, the provenance of the root > + * is unknown. Even if KVM ensures there are no stale TLB entries > + * for a freed root, in theory another hypervisor could have left > + * stale entries. Flushing on alloc also allows KVM to skip the TLB > + * flush when freeing a root (see kvm_tdp_mmu_put_root()). > + */ > static_call(kvm_x86_flush_tlb_current)(vcpu); > out: > return r; > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c > index b97a4125feac..921fa386df99 100644 > --- a/arch/x86/kvm/mmu/tdp_mmu.c > +++ b/arch/x86/kvm/mmu/tdp_mmu.c > @@ -93,7 +93,15 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root, > list_del_rcu(&root->link); > spin_unlock(&kvm->arch.tdp_mmu_pages_lock); > > - zap_gfn_range(kvm, root, 0, -1ull, false, false, shared); > + /* > + * A TLB flush is not necessary as KVM performs a local TLB flush when > + * allocating a new root (see kvm_mmu_load()), and when migrating vCPU > + * to a different pCPU. Note, the local TLB flush on reuse also > + * invalidates any paging-structure-cache entries, i.e. TLB entries for > + * intermediate paging structures, that may be zapped, as such entries > + * are associated with the ASID on both VMX and SVM. > + */ > + (void)zap_gfn_range(kvm, root, 0, -1ull, false, false, shared); Discussed offline with Sean. Now I get myself comfortable with the style of mmu with multiple 'roots' and leaving TLB unflushed for invalidated roots. I guess one minor improvement on the comment could be: "A TLB flush is not necessary as each vCPU performs a local TLB flush when allocating or assigning a new root (see kvm_mmu_load()), and when migrating to a different pCPU." The above could be better since "KVM performs a local TLB flush" makes readers think why we miss the 'remote' TLB flushes? > > call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback); > } > -- > 2.31.1 > >
