linux-kernel - Re: [v2 PATCH 1/2] powernv/powerpc:Save/Restore additional SPRs for stop4 cpuidle

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Date:   Wed, 19 Jul 2017 19:03:23 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 1/2] powernv/powerpc:Save/Restore additional SPRs for
 stop4 cpuidle

On Wed, 19 Jul 2017 13:48:49 +0530
"Gautham R. Shenoy" <ego@...ux.vnet.ibm.com> wrote:

> From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
> 
> The stop4 idle state on POWER9 is a deep idle state which loses
> hypervisor resources, but whose latency is low enough that it can be
> exposed via cpuidle.
> 
> Until now, the deep idle states which lose hypervisor resources (eg:
> winkle) were only exposed via CPU-Hotplug.  Hence currently on wakeup
> from such states, barring a few SPRs which need to be restored to
> their older value, rest of the SPRS are reinitialized to their values
> corresponding to that at boot time.
> 
> When stop4 is used in the context of cpuidle, we want these additional
> SPRs to be restored to their older value, to ensure that the context
> on the CPU coming back from idle is same as it was before going idle.
> 
> In this patch, we define a SPR save area in PACA (since we have used
> up the volatile register space in the stack) and on POWER9, we restore
> SPRN_PID, SPRN_LDBAR, SPRN_FSCR, SPRN_HFSCR, SPRN_MMCRA, SPRN_MMCR1,
> SPRN_MMCR2 to the values they had before entering stop.
> 
> Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> ---
>  arch/powerpc/include/asm/paca.h   |  7 ++++++
>  arch/powerpc/kernel/asm-offsets.c | 12 ++++++++++
>  arch/powerpc/kernel/idle_book3s.S | 46 +++++++++++++++++++++++++++++++++++++--
>  3 files changed, 63 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
> index dc88a31..a6b9ea6 100644
> --- a/arch/powerpc/include/asm/paca.h
> +++ b/arch/powerpc/include/asm/paca.h
> @@ -48,6 +48,7 @@
>  #define get_lppaca()	(get_paca()->lppaca_ptr)
>  #define get_slb_shadow()	(get_paca()->slb_shadow_ptr)
>  
> +#define MAX_STOP_SPRS     7
>  struct task_struct;
>  
>  /*
> @@ -183,6 +184,12 @@ struct paca_struct {
>  	struct paca_struct **thread_sibling_pacas;
>  	/* The PSSCR value that the kernel requested before going to stop */
>  	u64 requested_psscr;
> +
> +	/*
> +	 * Save area for additional SPRs that need to be
> +	 * saved/restored during cpuidle stop.
> +	 */
> +	u64 stop_spr_save_area[MAX_STOP_SPRS];
>  #endif
>  
>  #ifdef CONFIG_PPC_STD_MMU_64
> diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
> index a7b5af3..0262283 100644
> --- a/arch/powerpc/kernel/asm-offsets.c
> +++ b/arch/powerpc/kernel/asm-offsets.c
> @@ -743,6 +743,18 @@ int main(void)
>  	OFFSET(PACA_SUBCORE_SIBLING_MASK, paca_struct, subcore_sibling_mask);
>  	OFFSET(PACA_SIBLING_PACA_PTRS, paca_struct, thread_sibling_pacas);
>  	OFFSET(PACA_REQ_PSSCR, paca_struct, requested_psscr);
> +
> +	OFFSET(PACA_PID, paca_struct, stop_spr_save_area[0]);
> +	OFFSET(PACA_LDBAR, paca_struct, stop_spr_save_area[1]);
> +	OFFSET(PACA_FSCR, paca_struct, stop_spr_save_area[2]);
> +	OFFSET(PACA_HFSCR, paca_struct, stop_spr_save_area[3]);
> +
> +	/* On POWER9, we are already saving MMCR0 for ESL=EC=1 */
> +	OFFSET(PACA_MMCRA, paca_struct, stop_spr_save_area[4]);
> +	OFFSET(PACA_MMCR1, paca_struct, stop_spr_save_area[5]);
> +	OFFSET(PACA_MMCR2, paca_struct, stop_spr_save_area[6]);

Don't these offset names go against convention?

Look at e.g., how PACA_EXGEN is used. I would prefer using that
convention. You could make the name slightly shorter too, e.g.,
just stop_sprs or so.


> @@ -791,8 +834,7 @@ no_segments:
>  	bctrl
>  
>  BEGIN_FTR_SECTION
> -	ld	r4,_LPCR(r1)
> -	mtspr	SPRN_LPCR,r4
> +	bl 	power9_restore_additional_sprs
>  END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
>  hypervisor_state_restored:
>  

Also perhaps just a little comment in the code explaining why
power8 does not need this because it only uses it for hotplug
as you do in the changelog would help when reading the code.

Sorry I didn't pick this up on your v1 patches. If you change
those minor things, please add

Reviewed-by: Nicholas Piggin <npiggin@...il.com>

Thanks,
Nick