x86/cpuid: Disentangle logic for new feature leaves

x86/cpuid: Disentangle logic for new feature leaves

Adding a new feature leaf is a reasonable amount of boilerplate and for the
patch to build, at least one feature from the new leaf needs defining.  This
typically causes two non-trivial changes to be merged together.

First, have gen-cpuid.py write out some extra placeholder defines:

  #define CPUID_BITFIELD_11 bool :1, :1, lfence_dispatch:1, ...
  #define CPUID_BITFIELD_12 uint32_t :32 /* placeholder */
  #define CPUID_BITFIELD_13 uint32_t :32 /* placeholder */
  #define CPUID_BITFIELD_14 uint32_t :32 /* placeholder */
  #define CPUID_BITFIELD_15 uint32_t :32 /* placeholder */

This allows DECL_BITFIELD() to be added to struct cpuid_policy without
requiring a XEN_CPUFEATURE() declared for the leaf.  The choice of 4 is
arbitrary, and allows us to add more than one leaf at a time if necessary.

Second, rework generic_identify() to not use specific feature names.

The choice of deriving the index from a feature was to avoid mismatches, but
its correctness depends on bugs like c/s 249e0f1d8f20 ("x86/cpuid: Fix
TSXLDTRK definition") not happening.

Switch to using FEATURESET_* just like the policy/featureset helpers.  This
breaks the cognitive complexity of needing to know which leaf a specifically
named feature should reside in, and is shorter to write.  It is also far
easier to identify as correct at a glance, given the correlation with the
CPUID leaf being read.

In addition, tidy up some other bits of generic_identify()
 * Drop leading zeros from leaf numbers.
 * Don't use a locked update for X86_FEATURE_APERFMPERF.
 * Rework extended_cpuid_level calculation to avoid setting it twice.
 * Use "leaf >= $N" consistently so $N matches with the CPUID input.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>