Re: [PATCH v2 1/5] powerpc64/ftrace: Add a field in paca to disable ftrace in unsafe code paths

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Naveen N. Rao wrote:
> We have some C code that we call into from real mode where we cannot
> take any exceptions. Though the C functions themselves are mostly safe,
> if these functions are traced, there is a possibility that we may take
> an exception. For instance, in certain conditions, the ftrace code uses
> WARN(), which uses a 'trap' to do its job.
>=20
> For such scenarios, introduce a new field in paca 'ftrace_disabled',
> which is checked on ftrace entry before continuing. This field can then
> be set to a non-zero value to disable/pause ftrace, and reset to zero to
> resume ftrace.
>=20
> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> ---
> Changes since v1:
> - Do not guard the paca field check within CONFIG_KVM.
> - Move the early return code out of the usual path to optimize for=20
>   better cache performance in the normal case when ftrace is enabled.
>=20
>  arch/powerpc/include/asm/paca.h                |  1 +
>  arch/powerpc/kernel/asm-offsets.c              |  1 +
>  arch/powerpc/kernel/trace/ftrace_64_mprofile.S | 14 ++++++++++++++
>  arch/powerpc/kernel/trace/ftrace_64_pg.S       |  4 ++++
>  4 files changed, 20 insertions(+)
>=20
> diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/p=
aca.h
> index d2bf71dddbef..4f47adc2a408 100644
> --- a/arch/powerpc/include/asm/paca.h
> +++ b/arch/powerpc/include/asm/paca.h
> @@ -211,6 +211,7 @@ struct paca_struct {
>  	u16 in_mce;
>  	u8 hmi_event_available;		/* HMI event is available */
>  	u8 hmi_p9_special_emu;		/* HMI P9 special emulation */
> +	u8 ftrace_disabled;
>  #endif
> =20
>  	/* Stuff for accurate time accounting */
> diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-=
offsets.c
> index ea5eb91b836e..8e4fc96ff6bc 100644
> --- a/arch/powerpc/kernel/asm-offsets.c
> +++ b/arch/powerpc/kernel/asm-offsets.c
> @@ -240,6 +240,7 @@ int main(void)
>  	OFFSET(PACA_RFI_FLUSH_FALLBACK_AREA, paca_struct, rfi_flush_fallback_ar=
ea);
>  	OFFSET(PACA_EXRFI, paca_struct, exrfi);
>  	OFFSET(PACA_L1D_FLUSH_SIZE, paca_struct, l1d_flush_size);
> +	OFFSET(PACA_FTRACE_DISABLED, paca_struct, ftrace_disabled);
> =20
>  #endif
>  	OFFSET(PACAHWCPUID, paca_struct, hw_cpu_id);
> diff --git a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S b/arch/powerp=
c/kernel/trace/ftrace_64_mprofile.S
> index 3f3e81852422..8f2380304ef1 100644
> --- a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
> +++ b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
> @@ -47,6 +47,12 @@ _GLOBAL(ftrace_caller)
>  	/* Save all gprs to pt_regs */
>  	SAVE_GPR(0, r1)
>  	SAVE_10GPRS(2, r1)
> +
> +	/* Ok to continue? */
> +	lbz	r3, PACA_FTRACE_DISABLED(r13)
> +	cmpdi	r3, 0
> +	beq	ftrace_no_trace
	^^^ bne

:facepalm:

- Naveen

=