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

Nicholas Piggin [off-list ref] writes:

On Wed, 19 Jul 2017 13:48:49 +0530
"Gautham R. Shenoy" [off-list ref] wrote:

quoted

From: "Gautham R. Shenoy" <redacted>

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 <redacted>
---
 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.

Yes please.

If I see PACA_MMCRA I'm expecting that's paca->mmcra.

Also if the same values always go in the same place then please use a
proper struct, rather than an array. ie.

struct stop_sprs
{
	u64 pid;
        u64 ldbar;
        ...
}

cheers