Re: [RFC PATCH] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler

[Will Deacon]

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