Re: [PATCH v2 1/2] kasan/hw-tags: introduce kasan.store_only option

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Aug 15, 2025 at 04:10:59PM +0100, Yeoreum Yun wrote:
> > > Like we do in mte_enable_kernel_asymm(), if the feature is not available
> > > just fall back to checking both reads and writes in the chosen
> > > async/sync/asymm way. You can add some pr_info() to inform the user of
> > > the chosen kasan mode. It's really mostly an performance choice.
> >
> > But MTE_STORE_ONLY is defined as a SYSTEM_FEATURE.
> > This means that when it is called from kasan_init_hw_tags_cpu(),
> > the store_only mode is never set in system_capability,
> > so it cannot be checked using cpus_have_cap().
> >
> > Although the MTE_STORE_ONLY capability is verified by
> > directly reading the ID register (seems ugly),
> > my concern is the potential for an inconsistent state across CPUs.
> >
> > For example, in the case of ASYMM, which is a BOOT_CPU_FEATURE,
> > all CPUs operate in the same mode —
> > if ASYMM is not supported, either
> > all CPUs run in synchronous mode, or all run in asymmetric mode.
> >
> > However, for MTE_STORE_ONLY, CPUs that support the feature will run in store-only mode,
> > while those that do not will run with full checking for all operations.
> >
> > If we want to enable MTE_STORE_ONLY in kasan_init_hw_tags_cpu(),
> > I believe it should be reclassified as a BOOT_CPU_FEATURE.x
> > Otherwise, the cpu_enable_mte_store_only() function should still be called
> > as the enable callback for the MTE_STORE_ONLY feature.
> > In that case, kasan_enable_store_only() should be invoked (remove late init),
> > and if it returns an error, stop_machine() should be called to disable
> > the STORE_ONLY feature on all other CPUs
> > if any CPU is found to lack support for MTE_STORE_ONLY.
> >
> > Am I missing something?

Good point.

> So, IMHO like the ASYMM feature, it would be good to change
> MTE_STORE_ONLY as BOOT_CPU_FEATURE.
> That would makes everything as easiler and clear.

Yeah, let's do this. If people mix different features, we'll revisit at
that time. The asymmetric tag checking is also a boot CPU feature.

-- 
Catalin