Re: [PATCH 1/2] arm64: capabilities: Allow flexibility in scope

[Suzuki K Poulose]

On 08/02/18 16:10, Dave Martin wrote:
> On Thu, Feb 08, 2018 at 12:12:37PM +0000, Suzuki K Poulose wrote:
> > So far we have restricted the scopes for the capabilities
> > as follows :
> >   1) Errata workaround check are run all CPUs (i.e, always
> >      SCOPE_LOCAL_CPU)
> >   2) Arm64 features are run only once after the sanitised
> >      feature registers are available using the SCOPE_SYSTEM.
> >
> > This prevents detecting cpu features that can be detected
> > on one or more CPUs with SCOPE_LOCAL_CPU (e.g KPTI). Similarly
> > for errata workaround with SCOPE_SYSTEM.
> >
> > This patch makes sure that we allow flexibility of having
> > any scope for a capability. So, we now run through both
> > arm64_features and arm64_errata in two phases for detection:
> >
> >   a) with SCOPE_LOCAL_CPU filter on each boot time enabled
> >      CPUs.
> >   b) with SCOPE_SYSTEM filter only once after all boot time
> >      enabled CPUs are active.


> >   
> >   static void update_cpu_ftr_reg(struct arm64_ftr_reg *reg, u64 new)
> > @@ -1387,13 +1389,15 @@ static void verify_local_cpu_errata_workarounds(void)
> >   static void update_cpu_errata_workarounds(void)
> >   {
> >   	update_cpu_capabilities(arm64_errata,
> > -				ARM64_CPUCAP_SCOPE_ALL,
> > +				ARM64_CPUCAP_SCOPE_LOCAL_CPU,
>
> In isolation, this looks strange because it seems to handle only a
> subset of arm64_errata now.
>
> I think I understand the change as follows:
>
>   * all previously-existing errata workarounds SCOPE_LOCAL_CPU
>     anyway, so the behavior here doesn't change for any existing
>     caps;
>
>   * the non SCOPE_SYSTEM workarounds (which this patch prepares for)
"the SCOPE_SYSTEM" workarounds..."

>     are handled by the new setup_errata_workaround() path.

> Similarly, the features handling is split into two: one mirroring
> the current behaviour (for SCOPE_SYSTEM this time) and one handling> the rest, for supporting SCOPE_CPU_LOCAL features in subsequent
> patches.

Right. The changes here are :

New behavior:
  - Run SCOPE_LOCAL_CPU filter on arm64_features on all CPUs (newly added with
    this patch) via (newly added)update_cpu_local_features().

Split of existing behavior:
  - Run SCOPE_LOCAL_CPU(instead of the earlier SCOPE_ALL) on all CPUs in
    update_cpu_errata_workarounds()
  - Run SCOPE_SYSTEM filter on arm64_errata, once, via setup_errata_workarounds()

> Does that sound right?
>
> I'm not sure we need extra comments or documentation; I just want
> to check that I've understood the patch correctly.

So, would you prefer this split to the original patch ?

Cheers
Suzuki