Re: [PATCH] arm64: Defer the GMID_EL1 read to {init,update}_cpu_features()

[Marc Zyngier <maz@kernel.org>]

On Wed, 19 Aug 2026 15:25:58 +0100,
Fuad Tabba <fuad.tabba@linux.dev> wrote:
> 
> __cpuinfo_store_cpu() gates the GMID_EL1 read on the raw
> ID_AA64PFR1_EL1, so it reads the register on MTE hardware even when the
> kernel has disabled MTE (CONFIG_ARM64_MTE=n or arm64.nomte). KVM sets
> HCR_EL2.TID5 in that case, which traps the read to EL2 and injects an
> UNDEF:
> 
>   Internal error: Oops - Undefined instruction: 0000000002000000 [#1]
>   pc : __cpuinfo_store_cpu+0xf4/0x264
>   Call trace:
>    __cpuinfo_store_cpu+0xf4/0x264 (P)
>    secondary_start_kernel+0xc8/0x1d0
>    __secondary_switched+0xc0/0xc4
>   Kernel panic - not syncing: Attempted to kill the idle task!
> 
> Only pKVM is affected, and only for a CPU onlined after KVM init: its

Is that for a CPU that has gone through a an offline/online cycle?
Because otherwise, such a CPU wouldn't be able to boot at all (we have
a strong requirement that pKVM sees all CPUs at boot time).

If that's the case, some clarification would be good. Otherwise, some
clarification is absolutely required! ;-)

> PSCI CPU_ON relay sets the host HCR before the CPU enters EL1, whereas
> plain nVHE sets it at CPUHP_AP_KVM_ONLINE, after cpuinfo_store_cpu().
> 
> Defer the read to {init,update}_cpu_features() and gate it on the
> sanitised ID register, as MPAM already does. system_supports_mte()
> cannot serve as the gate, as update_cpu_features() also runs during
> initial SMP bring-up, before smp_cpus_done() calls
> setup_system_features(). The init path gains the CONFIG_ARM64_MTE test
> the update path already had, leaving SYS_GMID_EL1 uninitialised when
> MTE is compiled out, where its only other user (lib/mte.S) is not
> built.

I'm starting to wonder whether having CPUs to go through a feature
collection process during an offline/online cycle is a good idea. The
data should still be there, and is not expected to change.

Is it just that we don't have the correct tracking information?

Thanks,

	M.

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