For testing, development, and experimentation, add the ability to force
the kernel to behave as if the CPU has a bug, even if it doesn't, using
a command-line param.

Also do this in general for CPU flags, since:

 - The infrastructure is the same so there is almost no extra
   implementation complexity.

 - While setting random CPU flags is certain to break the kernel in
   mysterious and horrifying ways, this is not dramatically worse than
   setting CPU bugs. Although CPU bug mitigations don't have any very
   obvious ways to break the system if run on the wrong hardware, it's
   still very much an unsupported configuration, even beyond the
   security concern implied breaking mitigation logic.

   Since a taint and scary docs are necessary regardless, supporting
   arbitrary CPU flags doesn't add significant maintenance/support
   burden either.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
Changes in v2:
- Switched from a bugs-only force_cpu_bug= to a more general setcpuid=.
- Made it taint the kernel.
- Made docs sound scarier.
- Spellchecked and avoided new usage of personal pronouns.
- Link to v1: https://lore.kernel.org/r/20241119-force-cpu-bug-v1-1-2aa31c6c1ccf@google.com

---
Brendan Jackman (3):
      x86/cpu: Create helper to parse clearcpuid param
      x86/cpu: Add setcpuid cmdline param
      x86/cpu: Enable modifying bug flags with {clear,set}puid

 arch/x86/include/asm/cpufeature.h |   1 +
 arch/x86/kernel/cpu/common.c      | 139 +++++++++++++++++++++++---------------
 2 files changed, 87 insertions(+), 53 deletions(-)
---
base-commit: eabcdba3ad4098460a376538df2ae36500223c1e
change-id: 20241119-force-cpu-bug-94a08ab0239f

Best regards,
-- 
Brendan Jackman <jackmanb@google.com>