Re: git: ddf0ed09bd8f - main - sdt: Implement SDT probes using hot-patching

On Wed, Jun 19, 2024 at 1:58 PM Mark Johnston <markj@freebsd.org> wrote:
>
> The branch main has been updated by markj:
>
> URL: https://cgit.FreeBSD.org/src/commit/?id=ddf0ed09bd8f83677407db36828aca2c10f419c9
>
> commit ddf0ed09bd8f83677407db36828aca2c10f419c9
> Author:     Mark Johnston <markj@FreeBSD.org>
> AuthorDate: 2024-06-19 20:57:09 +0000
> Commit:     Mark Johnston <markj@FreeBSD.org>
> CommitDate: 2024-06-19 20:57:41 +0000
>
>     sdt: Implement SDT probes using hot-patching
>
>     The idea here is to avoid a memory access and conditional branch per
>     probe site.  Instead, the probe is represented by an "unreachable"
>     unconditional function call.  asm goto is used to store the address of
>     the probe site (represented by a no-op sled) and the address of the
>     function call into a tracepoint record.  Each SDT probe carries a list
>     of tracepoints.

Questions out of curiosity and maybe ignorance:

How does this work with relocations?  Something must be adjusting these
addresses?

> +/*
> + * Work around an apparent clang bug or limitation which prevents the use of the
> + * "i" (immediate) constraint with the probe structure.
> + */
> +#define        _SDT_ASM_PROBE_CONSTRAINT       "Ws"
> +#define        _SDT_ASM_PROBE_OPERAND          "p"

Is it because i386 kmods are built with -fPIC?

By the way, it seems gcc13 (latest in ports) doesn't support the "Ws"
constraint.  It seems to have been added to gcc 14.  I know i386 is tier
2 these days, and gcc is a second consideration anyway.  Trying to test
out a patch for i386 gcc, I found that it doesn't build currently and
this is one of a few reasons.

Ryan