Using the kernel thread pointer on aarch64 (tpidr_el1)

Using the kernel thread pointer on aarch64 (tpidr_el1)

[Zach 1]

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Hi,

On AArch64, GCC seems to use tpidr_el0 (user-mode accessible) to access the
thread pointer for loading thread-local variables by default. I can't find
any option to change this to tpidr_el1 (or tpidr_el2/tpidr_el3). This is
necessary for using thread-local variables in kernel code. Clang has the
`-mtp` option for AArch64 (
https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mtp)
that controls this, but it seems GCC does not? Does any such option exist,
and if not is there any chance to add support for it? Thanks for any help.

Zach

RE: Using the kernel thread pointer on aarch64 (tpidr_el1)

[Kyrylo Tkachov]

Hello,

> -----Original Message-----
> From: Gcc-help <gcc-help-bounces+kyrylo.tkachov=arm.com@gcc.gnu.org>
> On Behalf Of Zach 1 via Gcc-help
> Sent: Monday, February 13, 2023 2:04 AM
> To: gcc-help@gcc.gnu.org
> Subject: Using the kernel thread pointer on aarch64 (tpidr_el1)
> 
> Hi,
> 
> On AArch64, GCC seems to use tpidr_el0 (user-mode accessible) to access
> the
> thread pointer for loading thread-local variables by default. I can't find
> any option to change this to tpidr_el1 (or tpidr_el2/tpidr_el3). This is
> necessary for using thread-local variables in kernel code. Clang has the
> `-mtp` option for AArch64 (
> https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-
> clang-mtp)
> that controls this, but it seems GCC does not? Does any such option exist,
> and if not is there any chance to add support for it? Thanks for any help.

Looks like this exists for AArch32 (the arm port of GCC):
https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html

AArch64 doesn't support but I suppose it wouldn't be hard to add given arm supports it and clang as well.
Could you file a bug report in Bugzilla to keep track of the request please?

Thanks,
Kyrill

> 
> Zach

Re: Using the kernel thread pointer on aarch64 (tpidr_el1)

[Zach 1]

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Thanks, I have filed a Bugzilla report here:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108779.

On Mon, Feb 13, 2023 at 4:43 AM Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
wrote:

> Hello,
>
> > -----Original Message-----
> > From: Gcc-help <gcc-help-bounces+kyrylo.tkachov=arm.com@gcc.gnu.org>
> > On Behalf Of Zach 1 via Gcc-help
> > Sent: Monday, February 13, 2023 2:04 AM
> > To: gcc-help@gcc.gnu.org
> > Subject: Using the kernel thread pointer on aarch64 (tpidr_el1)
> >
> > Hi,
> >
> > On AArch64, GCC seems to use tpidr_el0 (user-mode accessible) to access
> > the
> > thread pointer for loading thread-local variables by default. I can't
> find
> > any option to change this to tpidr_el1 (or tpidr_el2/tpidr_el3). This is
> > necessary for using thread-local variables in kernel code. Clang has the
> > `-mtp` option for AArch64 (
> > https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-
> > clang-mtp)
> > that controls this, but it seems GCC does not? Does any such option
> exist,
> > and if not is there any chance to add support for it? Thanks for any
> help.
>
> Looks like this exists for AArch32 (the arm port of GCC):
> https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html
>
> AArch64 doesn't support but I suppose it wouldn't be hard to add given arm
> supports it and clang as well.
> Could you file a bug report in Bugzilla to keep track of the request
> please?
>
> Thanks,
> Kyrill
>
> >
> > Zach
>