On Thu, 2022-11-17 at 10:43 -0800, David Matlack wrote: > On Thu, Nov 17, 2022 at 8:14 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> > wrote: > > > > A removed SPTE is never present, hence the "if" in kvm_tdp_mmu_map > > only fails in the exact same conditions that the earlier loop > > tested in order to issue a "break". So, instead of checking twice > > the > > condition (upper level SPTEs could not be created or was frozen), > > just > > exit the loop with a goto---the usual poor-man C replacement for > > RAII > > early returns. > > > > While at it, do not use the "ret" variable for return values of > > functions that do not return a RET_PF_* enum. This is clearer > > and also makes it possible to initialize ret to RET_PF_RETRY. > > > > Suggested-by: Robert Hoo <robert.hu@xxxxxxxxxxxxxxx> > > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > > --- > > arch/x86/kvm/mmu/tdp_mmu.c | 40 ++++++++++++++++++-------------- > > ------ > > 1 file changed, 19 insertions(+), 21 deletions(-) > > > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c > > b/arch/x86/kvm/mmu/tdp_mmu.c > > index e08596775427..771210ce5181 100644 > > --- a/arch/x86/kvm/mmu/tdp_mmu.c > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c > > @@ -1159,7 +1159,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, > > struct kvm_page_fault *fault) > > struct kvm *kvm = vcpu->kvm; > > struct tdp_iter iter; > > struct kvm_mmu_page *sp; > > - int ret; > > + int ret = RET_PF_RETRY; > > > > kvm_mmu_hugepage_adjust(vcpu, fault); > > > > @@ -1168,23 +1168,25 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, > > struct kvm_page_fault *fault) > > rcu_read_lock(); > > > > tdp_mmu_for_each_pte(iter, mmu, fault->gfn, fault->gfn + 1) > > { > > + int r; > > + > > if (fault->nx_huge_page_workaround_enabled) > > disallowed_hugepage_adjust(fault, > > iter.old_spte, iter.level); > > > > if (iter.level == fault->goal_level) > > break; > > > > - /* Step down into the lower level page table if it > > exists. */ > > - if (is_shadow_present_pte(iter.old_spte) && > > - !is_large_pte(iter.old_spte)) > > - continue; > > - > > /* > > * If SPTE has been frozen by another thread, just > > give up and > > * retry, avoiding unnecessary page table > > allocation and free. > > */ > > if (is_removed_spte(iter.old_spte)) > > - break; > > + goto retry; > > + > > + /* Step down into the lower level page table if it > > exists. */ > > + if (is_shadow_present_pte(iter.old_spte) && > > + !is_large_pte(iter.old_spte)) > > + continue; > > > > /* > > * The SPTE is either non-present or points to a > > huge page that > > @@ -1196,13 +1198,17 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, > > struct kvm_page_fault *fault) > > sp->nx_huge_page_disallowed = fault- > > >huge_page_disallowed; > > > > if (is_shadow_present_pte(iter.old_spte)) > > - ret = tdp_mmu_split_huge_page(kvm, &iter, > > sp, true); > > + r = tdp_mmu_split_huge_page(kvm, &iter, sp, > > true); > > else > > - ret = tdp_mmu_link_sp(kvm, &iter, sp, > > true); > > + r = tdp_mmu_link_sp(kvm, &iter, sp, true); > > Can this fix be squashed into [1]? It seems like a serious enough > bug. > If 2 threads race to update the same PTE, KVM will return -EBUSY out > to userspace from KVM_RUN, I think. I'm not sure about QEMU, but that > would be fatal for the VM in Vanadium. > > [1] > https://lore.kernel.org/kvm/20221109185905.486172-3-dmatlack@xxxxxxxxxx/ > I think in you patch it's all right, since then before kvm_tdp_mmu_map() returns, it must go through tdp_mmu_map_handle_target_level(), it returns RET_PF_* enum. > > > > - if (ret) { > > + /* > > + * Also force the guest to retry the access if the > > upper level SPTEs > > + * aren't in place. > > + */ > > + if (r) { > > tdp_mmu_free_sp(sp); > > - break; > > + goto retry; > > } > > > > if (fault->huge_page_disallowed && > > @@ -1213,18 +1219,10 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, > > struct kvm_page_fault *fault) > > } > > } > > > > - /* > > - * Force the guest to retry the access if the upper level > > SPTEs aren't > > - * in place, or if the target leaf SPTE is frozen by > > another CPU. > > - */ > > - if (iter.level != fault->goal_level || > > is_removed_spte(iter.old_spte)) { > > - rcu_read_unlock(); > > - return RET_PF_RETRY; > > - } > > - > > ret = tdp_mmu_map_handle_target_level(vcpu, fault, &iter); > > - rcu_read_unlock(); > > > > +retry: > > + rcu_read_unlock(); > > return ret; > > } > > > > -- > > 2.31.1 > >