Subject: Re: FEAT_CMOW

From: Marc Zyngier <[email-masked]>
Date: Thu, 19 Feb 2026 08:52:36 +0000

On Thu, 19 Feb 2026 05:21:02 +0000,
Manoj Ekbote <[email-masked]> wrote:
> 
> Hi,
> 
> While looking at the mainline v6.18.6 sources, I noticed that
> FEAT_CMOW does not appear in the arm64 CPU capability table
> (arch/arm64/kernel/cpufeature.c). Since CMOW is a mandatory
> architectural feature starting in Arm v8.8, I expected to see it in
> the cpufeature framework.  How is the cpu feature list typically
> updated for architectural extensions like this?

When we have a use for it. FEAT_CMOW has the potential to break the
ABI we already expose to userspace, so I don't immediately see why
we'd care about it.

> Is it done only when the kernel needs to make use of the feature, or
> is there a process to actively add all mandatory architecture
> features?

The Linux kernel is thankfully not a validation tool for the
architecture, and is not aligned to any particular version of the
architecture either. A "mandatory" feature is only a requirement for
implementations, not for SW.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.