Re: git: f05795e3f65f - main - arm64: write PID in CONTEXTIDR_EL1 on ctx switch

On 9 Sep 2024, at 17:05, Andrew Turner <andrew@FreeBSD.org> wrote:
> 
> The branch main has been updated by andrew:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=f05795e3f65f305cb770ae91d8e9c8f05d267e0d
> 
> commit f05795e3f65f305cb770ae91d8e9c8f05d267e0d
> Author:     Zachary Leaf <zachary.leaf@arm.com>
> AuthorDate: 2024-07-31 08:23:24 +0000
> Commit:     Andrew Turner <andrew@FreeBSD.org>
> CommitDate: 2024-09-09 16:03:19 +0000
> 
>    arm64: write PID in CONTEXTIDR_EL1 on ctx switch
> 
>    Debug and trace features such as Statistical Profiling Extension (SPE)
>    use the CONTEXTIDR_EL1 register to get the PID of the current process.
> 
>    Add a sysctl switch to toggle writing the current PID into this register
>    in the thread switcher.
> 
>    To make use of the feature, the following sysctl switch must be set:
> 
>        sysctl machdep.pid_in_contextidr=1
> 
>    Kernel code can also toggle the sysctl by writing directly to the global
>    var behind the sysctl arm64_pid_in_contextidr:
> 
>        extern bool arm64_pid_in_contextidr;
> 
>    Reviewed by:    andrew
>    Sponsored by:   Arm Ltd
>    Differential Revision:  https://reviews.freebsd.org/D46192
> ---
> sys/arm64/arm64/genassym.c    |  2 ++
> sys/arm64/arm64/swtch.S       | 17 +++++++++++++++++
> sys/arm64/arm64/sys_machdep.c |  6 ++++++
> 3 files changed, 25 insertions(+)
> 
> diff --git a/sys/arm64/arm64/genassym.c b/sys/arm64/arm64/genassym.c
> index a4db825e976c..5a20169d51c3 100644
> --- a/sys/arm64/arm64/genassym.c
> +++ b/sys/arm64/arm64/genassym.c
> @@ -58,6 +58,8 @@ ASSYM(PCB_TPIDRRO, offsetof(struct pcb, pcb_tpidrro_el0));
> ASSYM(PCB_ONFAULT, offsetof(struct pcb, pcb_onfault));
> ASSYM(PCB_FLAGS, offsetof(struct pcb, pcb_flags));
> 
> +ASSYM(PR_PID, offsetof(struct proc, p_pid));
> +
> ASSYM(SF_UC, offsetof(struct sigframe, sf_uc));
> 
> ASSYM(TD_PROC, offsetof(struct thread, td_proc));
> diff --git a/sys/arm64/arm64/swtch.S b/sys/arm64/arm64/swtch.S
> index 6af70ca839a0..3a2bf2cb5a7f 100644
> --- a/sys/arm64/arm64/swtch.S
> +++ b/sys/arm64/arm64/swtch.S
> @@ -55,6 +55,15 @@
> 999:
> .endm
> 
> +.macro pid_in_context_idr label
> + adrp x9, arm64_pid_in_contextidr
> + ldr x10, [x9, :lo12:arm64_pid_in_contextidr]

This loads a 64-bit value, not an 8-bit bool as the variable is
defined. Depending on the alignment of arm64_pid_in_contextidr this can
even fail to link, and this shows up in GitHub Actions with the
clang-12 and clang-13 jobs failing.

Jess