[question] [Cortex-A57] how to discover implementation defined system registers?

[Catalin Marinas]

On Tue, Sep 08, 2015 at 02:44:28AM +0300, Alexey Klimov wrote:
> I'm implementing module for Linux kernel that needs access to
> implementation defined system registers that described in section:
> "D7.2.78 S3_<op1>_<Cn>_<Cm>_<op2>, IMPLEMENTATION DEFINED registers"
> in ARMv8 spec.

Why?

> In specs for Cortex-A57 they are also described as
> implementation defined. Page 4-86, table 4-15 in Cortex-A57 spec ARM
> DDI0488G.
> 
> Is it correct understanding that because of implementation-defined
> nature all information about are they implemented or not goes on
> vendor shoulders and should be exposed to kernel through
> 1) Device tree or ACPI. And only if kernel finds node or entry in
> DT/ACPI about this it can trigger code/driver that needs these regs.
> 
> or
> 
> 2) I'm looking on undef_hook, {un,}register_undef_hook() code in
> arch/arm64/traps.c. Can kernel "probe" to access to impl defined regs
> using undefined instr exceptions and undef_hooks to figure out
> map/impl features list/array/whatever for each cpu during startup (i'm
> also checking Suzuki patches for exposing cpu features)? Maybe not all
> impl defined regs can be probed this way and I suspect it might affect
> KVM or guests in some way.

As above, why does the kernel need to access these registers?

> Registers in question are L2CTLR_EL1, CPUMERRSR_EL1, L2MERRSR_EL1.

Usually, non-secure (i.e. Linux) access to such registers is blocked by
the (secure) firmware (at least writing to them), so they are not of
much use to Linux.

> Also, does information about CPU part, revision, variant and
> implementer play some role here? For example, cpu implementations with
> revision less than 1 never support this regs or only 0x41 as cpu
> implementer can provide these list of impl defined regs.

Yes, the CPU MIDR is the only reliable indication of which auxiliary
registers you have but, as I said above, they are meant for firmware to
access and Linux shouldn't care about them (at least arm64 Linux).

-- 
Catalin