[PATCH v3 0/4] x86/cpu: Add facility to force-enable CPU caps and bugs

Brendan Jackman
2025-02-18T18:59:49Z

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 v3:
- Added pre-patch to cleanup some unnecessary macro usage.
- More cleanups to commentary & commit messages.
- Link to v2: https://lore.kernel.org/r/20241220-force-cpu-bug-v2-0-7dc71bce742a@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 (4):
x86/cpu: Remove some macros about feature names
x86/cpu: Create helper to parse clearcpuid param
x86/cpu: Add setcpuid cmdline param
x86/cpu: Enable modifying bug flags with {clear,set}cpuid

arch/x86/include/asm/cpufeature.h | 6 +-
arch/x86/kernel/cpu/common.c | 147 +++++++++++++++++++++++---------------
2 files changed, 91 insertions(+), 62 deletions(-)
---
base-commit: 83f8eec51fc484fe20f8d20171f6d450080c04ea
change-id: 20241119-force-cpu-bug-94a08ab0239f

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