[PATCH] arm64: Generate cpucaps.h

[dann frazier]

On Tue, Sep 21, 2021 at 10:09:12PM +0100, Suzuki K Poulose wrote:
> 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.

That seems to me to be the simplest approach, and I've verified it
addresses the problem. I'll prepare a patch. Thanks Suzuki.

  -dann