[PATCH] arm64: Generate cpucaps.h

[Suzuki K Poulose]

On 21/09/2021 19:35, Mark Brown wrote:
> On Tue, Sep 21, 2021 at 12:08:11PM -0600, dann frazier wrote:
>> On Wed, Apr 28, 2021 at 01:12:31PM +0100, Mark Brown wrote:
> 
>>> This will result in a renumbering and reordering of the existing constants,
>>> since they are all internal only the values should not be important. The
>>> reordering will impact the order in which some steps in enumeration handle
>>> features but the algorithm is not intended to depend on this and I haven't
>>> seen any issues when testing.
> 
>> Unfortunately I believe I've hit a regression[*] due to such an
>> ordering dependency. UNMAP_KERNEL_AT_EL0 currently needs to be
>> processed after WORKAROUND_CAVIUM_27456. ThunderX systems are
>> incompatible with KPTI, so unmap_kernel_at_el0() bails if
>> WORKAROUND_CAVIUM_27456 is set. Because of the sorting,
>> WORKAROUND_CAVIUM_27456 will not yet have been considered when
>> unmap_kernel_at_el0() checks for it, so the kernel tries to
>> run w/ KPTI - and quickly falls over.
> 
>> I've verified that reordering cpucaps to move WORKAROUND_CAVIUM_27456
>> just above UNMAP_KERNEL_AT_EL0 restores the old behavior. I'm not sure
>> of the right way to address this - perhaps unmap_kernel_at_el0() could
>> check cavium_erratum_27456_cpus[] directly instead of keying on the
>> ARM64_WORKAROUND_CAVIUM_27456 cap?

Given that these capabilities are LOCAL_CPU scope, and the systems in
question never have heterogeneous CPUs, you could replace

  cpus_have_const_cap() =>  this_cpu_has_cap()

in unmap_kernel_at_el0().

If they were heterogeneous, we would fail anyway, no matter what the
order was.

Suzuki