On 08/02/18 16:10, Dave Martin wrote:

On Thu, Feb 08, 2018 at 12:12:37PM +0000, Suzuki K Poulose wrote:

quoted

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.

quoted

  
  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