Messages in this thread Patch in this message | | | Date | Fri, 11 Dec 2020 10:00:27 +0000 | From | Will Deacon <> | Subject | Re: [RFC PATCH] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler |
| |
On Fri, Dec 11, 2020 at 09:49:28AM +0000, Marc Zyngier wrote: > On 2020-12-11 08:01, Yanan Wang wrote: > > @@ -461,25 +462,56 @@ static int stage2_map_set_prot_attr(enum > > kvm_pgtable_prot prot, > > return 0; > > } > > > > +static bool stage2_set_valid_leaf_pte_pre(u64 addr, u32 level, > > + kvm_pte_t *ptep, kvm_pte_t new, > > + struct stage2_map_data *data) > > +{ > > + kvm_pte_t old = *ptep, old_attr, new_attr; > > + > > + if ((old ^ new) & (~KVM_PTE_LEAF_ATTR_PERMS)) > > + return false; > > + > > + /* > > + * Skip updating if we are trying to recreate exactly the same mapping > > + * or to reduce the access permissions only. And update the valid leaf > > + * PTE without break-before-make if we are trying to add more access > > + * permissions only. > > + */ > > + old_attr = (old & KVM_PTE_LEAF_ATTR_PERMS) ^ > > KVM_PTE_LEAF_ATTR_HI_S2_XN; > > + new_attr = (new & KVM_PTE_LEAF_ATTR_PERMS) ^ > > KVM_PTE_LEAF_ATTR_HI_S2_XN; > > + if (new_attr <= old_attr) > > + return true; > > + > > + WRITE_ONCE(*ptep, new); > > + kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level); > > I think what bothers me the most here is that we are turning a mapping into > a permission update, which makes the code really hard to read, and mixes > two things that were so far separate. > > I wonder whether we should instead abort the update and simply take the > fault > again, if we ever need to do it.
That's a nice idea. If we could enforce that we don't alter permissions on the map path, and instead just return e.g. -EAGAIN then that would be a very neat solution and would cement the permission vs translation fault division.
Will
| |