[PATCH v6 25/26] arm64: KVM: Allow mapping of vectors outside of the RAM region

[Andrew Jones]

On Thu, Mar 15, 2018 at 06:47:22PM +0000, Marc Zyngier wrote:
> On 15/03/18 17:08, Andrew Jones wrote:
> > On Thu, Mar 15, 2018 at 04:17:53PM +0000, Marc Zyngier wrote:
> >> On 15/03/18 15:54, Andrew Jones wrote:
> >>> On Wed, Mar 14, 2018 at 04:50:48PM +0000, Marc Zyngier wrote:
> >>>>  static inline void *kvm_get_hyp_vector(void)
> >>>>  {
> >>>>  	struct bp_hardening_data *data = arm64_get_bp_hardening_data();
> >>>> -	void *vect = kvm_ksym_ref(__kvm_hyp_vector);
> >>>> +	void *vect = kern_hyp_va(kvm_ksym_ref(__kvm_hyp_vector));
> >>>> +	int slot = -1;
> >>>>  
> >>>> -	if (data->fn) {
> >>>> -		vect = __bp_harden_hyp_vecs_start +
> >>>> -		       data->hyp_vectors_slot * SZ_2K;
> >>>> +	if (cpus_have_const_cap(ARM64_HARDEN_BRANCH_PREDICTOR) && data->fn) {
> >>>
> >>> Now that I'm trying to consider heterogeneous systems, I'm wondering why
> >>> harden BP isn't checked with this_cpu_has_cap() like harden EL2 vectors.
> >>> I'm also wondering if it's even possible for CPUs to come up without all
> >>> of them having harden EL2 vectors if the boot CPU has it. Won't
> >>> verify_local_cpu_errata_workarounds() require it? I'm probably just
> >>> getting lost in all the capability stuff...
> >>
> >> Checking harden BP on this particular CPU will only tell you if this CPU
> >> is affected, but won't give you any additional information (i.e. you'd
> >> still need to check obtain the stuff pointed to by data).
> >>
> >> Checking for *all* CPUs in one go has some advantages: it is a static
> >> key, which means that unaffected platforms will fly (this is a hot path
> >> on VHE), and you can check data if you're affected.
> > 
> > Is there a problem with using the static key to check
> > ARM64_HARDEN_EL2_VECTORS below? (Sorry, I'm still confused why we're not
> > using the same functions for both.) 
> 
> Consider the following case: heterogeneous system with CPU0 that has
> HBP, and CPU1 that has HEL. CPU0 is using its own vector slot, and CPU1
> should be using an extra one (it doesn't have a dedicated slot).
> 
> When CPU0 claims its vectors, it will get the "normal" version of the
> mapping. And then, if we're using cpus_have_cap, we turn that into an
> out-of-line mapping. That's not completely wrong (the system still runs
> fine), but that's not the expected behaviour. If we restrict the second
> phase to this_cpu_has_cap(), we get the expected CPU0 using the normal
> mapping, and CPU1 does the exact opposite (which we also want).

Thanks for the additional explanation. I've finally got my head wrapped
around this and it looks good to me. And, I see now that my comment about
verify_local_cpu_errata_workarounds() only applies to CPUs plugged after
boot. The comment in check_local_cpu_capabilities() states clearly each
CPU present at boot gets a chance to update cpu_hwcaps - which also
clarifies why kvm_map_vectors() *must* use cpus_have_const_cap(), else
risk the el2 vector slot not being allocated.

Thanks,
drew