Subject: Re: [PATCH] powerpc/64s: relocation, register save fixes for system reset interrupt

From: Nicholas Piggin <[email-masked]>
Date: Wed, 2 Nov 2016 17:57:01 +1100

On Wed, 2 Nov 2016 11:34:59 +0530
Mahesh Jagannath Salgaonkar <[email-masked]> wrote:

> On 10/13/2016 07:47 AM, Nicholas Piggin wrote:
> > This patch does a couple of things. First of all, powernv immediately
> > explodes when running a relocated kernel, because the system reset
> > exception for handling sleeps does not do correct relocated branches.
> > 
> > Secondly, the sleep handling code trashes the condition and cfar
> > registers, which we would like to preserve for debugging purposes (for
> > non-sleep case exception).
> > 
> > This patch changes the exception to use the standard format that saves
> > registers before any tests or branches are made. It adds the test for
> > idle-wakeup as an "extra" to break out of the normal exception path.
> > Then it branches to a relocated idle handler that calls the various
> > idle handling functions.
> > 
> > After this patch, POWER8 CPU simulator now boots powernv kernel that is
> > running at non-zero.
> > 
> > Cc: Balbir Singh <[email-masked]>
> > Cc: Shreyas B. Prabhu <[email-masked]>
> > Cc: Gautham R. Shenoy <[email-masked]>
> > Signed-off-by: Nicholas Piggin <[email-masked]>
> > ---
> >  arch/powerpc/include/asm/exception-64s.h | 16 ++++++++++
> >  arch/powerpc/kernel/exceptions-64s.S     | 50 ++++++++++++++++++--------------
> >  2 files changed, 45 insertions(+), 21 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/exception-64s.h b/arch/powerpc/include/asm/exception-64s.h
> > index 2e4e7d8..84d49b1 100644
> > --- a/arch/powerpc/include/asm/exception-64s.h
> > +++ b/arch/powerpc/include/asm/exception-64s.h
> > @@ -93,6 +93,10 @@
> >  	ld	reg,PACAKBASE(r13);	/* get high part of &label */	\
> >  	ori	reg,reg,(FIXED_SYMBOL_ABS_ADDR(label))@l;
> > 
> > +#define __LOAD_HANDLER(reg, label)					\
> > +	ld	reg,PACAKBASE(r13);					\
> > +	ori	reg,reg,(ABS_ADDR(label))@l;
> > +
> >  /* Exception register prefixes */
> >  #define EXC_HV	H
> >  #define EXC_STD
> > @@ -208,6 +212,18 @@ END_FTR_SECTION_NESTED(ftr,ftr,943)
> >  #define kvmppc_interrupt kvmppc_interrupt_pr
> >  #endif
> > 
> > +#ifdef CONFIG_RELOCATABLE
> > +#define BRANCH_TO_COMMON(reg, label)					\
> > +	__LOAD_HANDLER(reg, label);					\
> > +	mtctr	reg;							\
> > +	bctr
> > +
> > +#else
> > +#define BRANCH_TO_COMMON(reg, label)					\
> > +	b	label
> > +
> > +#endif
> > +
> >  #define __KVM_HANDLER_PROLOG(area, n)					\
> >  	BEGIN_FTR_SECTION_NESTED(947)					\
> >  	ld	r10,area+EX_CFAR(r13);					\
> > diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
> > index 08992f8..e680e84 100644
> > --- a/arch/powerpc/kernel/exceptions-64s.S
> > +++ b/arch/powerpc/kernel/exceptions-64s.S
> > @@ -95,19 +95,35 @@ __start_interrupts:
> >  /* No virt vectors corresponding with 0x0..0x100 */
> >  EXC_VIRT_NONE(0x4000, 0x4100)
> > 
> > -EXC_REAL_BEGIN(system_reset, 0x100, 0x200)
> > -	SET_SCRATCH0(r13)
> > +
> >  #ifdef CONFIG_PPC_P7_NAP
> > -BEGIN_FTR_SECTION
> > -	/* Running native on arch 2.06 or later, check if we are
> > -	 * waking up from nap/sleep/winkle.
> > +	/*
> > +	 * If running native on arch 2.06 or later, check if we are waking up
> > +	 * from nap/sleep/winkle, and branch to idle handler.
> >  	 */
> > -	mfspr	r13,SPRN_SRR1
> > -	rlwinm.	r13,r13,47-31,30,31
> > -	beq	9f
> > +#define IDLETEST(n)							\
> > +	BEGIN_FTR_SECTION ;						\
> > +	mfspr	r10,SPRN_SRR1 ;						\
> > +	rlwinm.	r10,r10,47-31,30,31 ;					\
> > +	beq-	1f ;							\
> > +	cmpwi	cr3,r10,2 ;						\
> > +	BRANCH_TO_COMMON(r10, system_reset_idle_common) ;		\
> > +1:									\
> > +	END_FTR_SECTION_IFSET(CPU_FTR_HVMODE | CPU_FTR_ARCH_206)
> > +#else
> > +#define IDLETEST NOTEST
> > +#endif
> > 
> > -	cmpwi	cr3,r13,2
> > -	GET_PACA(r13)
> > +EXC_REAL_BEGIN(system_reset, 0x100, 0x200)
> > +	SET_SCRATCH0(r13)
> > +	EXCEPTION_PROLOG_PSERIES(PACA_EXGEN, system_reset_common, EXC_STD,
> > +				 IDLETEST, 0x100)  
> 
> Very sorry for late review. On arch 2.07 and less if we wakeup from
> winkle then last bit of HSPGR0 would be set to 1. Hence before we access
> paca we need to fix it by clearing that bit and that is done in
> pnv_restore_hyp_resource(). But with this patch, we would end up there
> after going through EXCEPTION_PROLOG_PSERIES(). This macro gets the paca
> using GET_PACA(r13) and all the EXCEPTION_PROLOG_* starts
> using/accessing r13/paca without fixing it. Wouldn't this break things
> badly on arch 2.07 and less ? Am I missing anything ?

Arg, that's a stupid bug :( Thanks for catching it.

Would something like the following do the trick, do you think? I obviously
was not reaching winkle state in my testing.

Thanks,
Nick

---
 arch/powerpc/include/asm/exception-64s.h | 13 +++++++++++--
 arch/powerpc/kernel/exceptions-64s.S     | 11 ++++++++---
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/exception-64s.h b/arch/powerpc/include/asm/exception-64s.h
index 84d49b1..3ce4366 100644
--- a/arch/powerpc/include/asm/exception-64s.h
+++ b/arch/powerpc/include/asm/exception-64s.h
@@ -158,14 +158,17 @@ BEGIN_FTR_SECTION_NESTED(943)						\
 	std	ra,offset(r13);						\
 END_FTR_SECTION_NESTED(ftr,ftr,943)
 
-#define EXCEPTION_PROLOG_0(area)					\
-	GET_PACA(r13);							\
+#define EXCEPTION_PROLOG_0_PACA(area)					\
 	std	r9,area+EX_R9(r13);	/* save r9 */			\
 	OPT_GET_SPR(r9, SPRN_PPR, CPU_FTR_HAS_PPR);			\
 	HMT_MEDIUM;							\
 	std	r10,area+EX_R10(r13);	/* save r10 - r12 */		\
 	OPT_GET_SPR(r10, SPRN_CFAR, CPU_FTR_CFAR)
 
+#define EXCEPTION_PROLOG_0(area)					\
+	GET_PACA(r13);							\
+	EXCEPTION_PROLOG_0_PACA(area)
+
 #define __EXCEPTION_PROLOG_1(area, extra, vec)				\
 	OPT_SAVE_REG_TO_PACA(area+EX_PPR, r9, CPU_FTR_HAS_PPR);		\
 	OPT_SAVE_REG_TO_PACA(area+EX_CFAR, r10, CPU_FTR_CFAR);		\
@@ -196,6 +199,12 @@ END_FTR_SECTION_NESTED(ftr,ftr,943)
 	EXCEPTION_PROLOG_1(area, extra, vec);				\
 	EXCEPTION_PROLOG_PSERIES_1(label, h);
 
+/* Have the PACA in r13 already */
+#define EXCEPTION_PROLOG_PSERIES_PACA(area, label, h, extra, vec)	\
+	EXCEPTION_PROLOG_0_PACA(area);					\
+	EXCEPTION_PROLOG_1(area, extra, vec);				\
+	EXCEPTION_PROLOG_PSERIES_1(label, h);
+
 #define __KVMTEST(h, n)							\
 	lbz	r10,HSTATE_IN_GUEST(r13);				\
 	cmpwi	r10,0;							\
diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 08ba447..1ba82ea 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -116,7 +116,9 @@ EXC_VIRT_NONE(0x4000, 0x4100)
 
 EXC_REAL_BEGIN(system_reset, 0x100, 0x200)
 	SET_SCRATCH0(r13)
-	EXCEPTION_PROLOG_PSERIES(PACA_EXGEN, system_reset_common, EXC_STD,
+	GET_PACA(r13)
+	clrrdi	r13,r13,1 /* Last bit of HSPRG0 is set if waking from winkle */
+	EXCEPTION_PROLOG_PSERIES_PACA(PACA_EXGEN, system_reset_common, EXC_STD,
 				 IDLETEST, 0x100)
 
 EXC_REAL_END(system_reset, 0x100, 0x200)
@@ -124,6 +126,9 @@ EXC_VIRT_NONE(0x4100, 0x4200)
 
 #ifdef CONFIG_PPC_P7_NAP
 EXC_COMMON_BEGIN(system_reset_idle_common)
+BEGIN_FTR_SECTION
+	GET_PACA(r13) /* Restore HSPRG0 to get the winkle bit in r13 */
+END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_300)
 	bl	pnv_restore_hyp_resource
 
 	li	r0,PNV_THREAD_RUNNING
@@ -169,7 +174,7 @@ EXC_REAL_BEGIN(machine_check, 0x200, 0x300)
 	SET_SCRATCH0(r13)		/* save r13 */
 	/*
 	 * Running native on arch 2.06 or later, we may wakeup from winkle
-	 * inside machine check. If yes, then last bit of HSPGR0 would be set
+	 * inside machine check. If yes, then last bit of HSPRG0 would be set
 	 * to 1. Hence clear it unconditionally.
 	 */
 	GET_PACA(r13)
@@ -388,7 +393,7 @@ EXC_COMMON_BEGIN(machine_check_handle_early)
 	/*
 	 * Go back to winkle. Please note that this thread was woken up in
 	 * machine check from winkle and have not restored the per-subcore
-	 * state. Hence before going back to winkle, set last bit of HSPGR0
+	 * state. Hence before going back to winkle, set last bit of HSPRG0
 	 * to 1. This will make sure that if this thread gets woken up
 	 * again at reset vector 0x100 then it will get chance to restore
 	 * the subcore state.
-- 
2.9.3