Re: [PATCH 0/3 v3] [AArch64] Support tagged pointer

Pedro Alves
2018-04-24T11:48:53Z

Hi,

On 04/20/2018 03:33 PM, Omair Javaid wrote:

> Pointer tagging information is stored in MMU registers so in linux
> user-space we cannot actually read if pointer tagging is enabled or not
> based on register bits.
> JTAG debuggers should be able to read MMU registers and know whether
> pointer tagging is enabled or not.
>
> Rationale behind adding a separate command is to allow other application to
> control pointer tagging for example bare-metal (non-linux OSes) which want
> to use pointer tagging can enable it. I must admit I dont know of any such
> use-case as of now.

Alright, that's in line with what I was thinking. Though, bare metal
should have access to MMU registers too. Ideally, things would Just Work
without user intervention. But I don't mind starting by adding a
user-controllable knob, it might be a convenient escape hatch. We can always
extend it from "on/off" -> "on/off/auto" setting, with auto the default
in future.

> Also I am not sure about the timeline of Linux Kernel patches going into
> gdb and for now I thought of this command as the most suitable option.
> Moreover some users might also be interested in combination where pointer
> tagging is enabled but Linux Kernel threads support is disabled so I
> thought we should give the control to the user in cases where we cannot
> predict use-cases.

If everyone agrees that proper Linux kernel support benefits from
its own osabi setting/name, then I don't see why we couldn't start by
adding the osabi setting as soon as we have a use for it, even if
the larger Linux Kernel patches aren't ready yet.

Thanks,
Pedro Alves