Re: [PATCH V2 8/8] powerpc: Add the ability to save VSX without giving it up

On Fri, 15 Jan 2016 17:25:26 +1100
Michael Neuling [off-list ref] wrote:

On Fri, 2016-01-15 at 16:04 +1100, Cyril Bur wrote:

quoted

This patch adds the ability to be able to save the VSX registers to
the
thread struct without giving up (disabling the facility) next time
the
process returns to userspace.

This patch builds on a previous optimisation for the FPU and VEC
registers
in the thread copy path to avoid a possibly pointless reload of VSX
state.

Signed-off-by: Cyril Bur <redacted>
---
 arch/powerpc/include/asm/switch_to.h |  1 -
 arch/powerpc/kernel/ppc_ksyms.c      |  4 ----
 arch/powerpc/kernel/process.c        | 23 ++++++++++++++++++-----
 arch/powerpc/kernel/vector.S         | 17 -----------------
 4 files changed, 18 insertions(+), 27 deletions(-)

diff --git a/arch/powerpc/include/asm/switch_to.h

b/arch/powerpc/include/asm/switch_to.h
index 29dda9d..4dfcd3e 100644

--- a/arch/powerpc/include/asm/switch_to.h
+++ b/arch/powerpc/include/asm/switch_to.h

@@ -52,7 +52,6 @@ static inline void disable_kernel_altivec(void)
 extern void enable_kernel_vsx(void);
 extern void flush_vsx_to_thread(struct task_struct *);
 extern void giveup_vsx(struct task_struct *);
-extern void __giveup_vsx(struct task_struct *);
 static inline void disable_kernel_vsx(void)
 {
 	msr_check_and_clear(MSR_FP|MSR_VEC|MSR_VSX);

diff --git a/arch/powerpc/kernel/ppc_ksyms.c

b/arch/powerpc/kernel/ppc_ksyms.c
index 41e1607..ef7024da 100644

--- a/arch/powerpc/kernel/ppc_ksyms.c
+++ b/arch/powerpc/kernel/ppc_ksyms.c

@@ -28,10 +28,6 @@ EXPORT_SYMBOL(load_vr_state);
 EXPORT_SYMBOL(store_vr_state);
 #endif
 
-#ifdef CONFIG_VSX
-EXPORT_SYMBOL_GPL(__giveup_vsx);
-#endif
-
 #ifdef CONFIG_EPAPR_PARAVIRT
 EXPORT_SYMBOL(epapr_hypercall_start);
 #endif

diff --git a/arch/powerpc/kernel/process.c

b/arch/powerpc/kernel/process.c
index 5566c32..3d907b8 100644

--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c

@@ -252,20 +252,33 @@ EXPORT_SYMBOL_GPL(flush_altivec_to_thread);
 #endif /* CONFIG_ALTIVEC */
 
 #ifdef CONFIG_VSX
-void giveup_vsx(struct task_struct *tsk)
+void __giveup_vsx(struct task_struct *tsk)
 {
-	check_if_tm_restore_required(tsk);
-
-	msr_check_and_set(MSR_FP|MSR_VEC|MSR_VSX);
 	if (tsk->thread.regs->msr & MSR_FP)
 		__giveup_fpu(tsk);
 	if (tsk->thread.regs->msr & MSR_VEC)
 		__giveup_altivec(tsk);
+	tsk->thread.regs->msr &= ~MSR_VSX;
+}
+
+void giveup_vsx(struct task_struct *tsk)
+{
+	check_if_tm_restore_required(tsk);
+
+	msr_check_and_set(MSR_FP|MSR_VEC|MSR_VSX);
 	__giveup_vsx(tsk);
 	msr_check_and_clear(MSR_FP|MSR_VEC|MSR_VSX);
 }
 EXPORT_SYMBOL(giveup_vsx);
 
+void save_vsx(struct task_struct *tsk)
+{
+	if (tsk->thread.regs->msr & MSR_FP)
+		save_fpu(tsk);
+	if (tsk->thread.regs->msr & MSR_VEC)
+		save_altivec(tsk);
+}
+
 void enable_kernel_vsx(void)
 {
 	WARN_ON(preemptible());

@@ -465,7 +478,7 @@ void save_all(struct task_struct *tsk)
 #endif
 #ifdef CONFIG_VSX
 	if (usermsr & MSR_VSX)
-		__giveup_vsx(tsk);
+		save_vsx(tsk);  

This seems suboptimal.  save_vsx() will call save_fpu() and
save_altivec() again, which you just called earlier in save_all().

Ah yes, will fix

save_vsx() is only used here, so could be static. 

Thanks.

Also, put the #ifdef junk as part of the function so that the caller
doesn't have to deal with it. 

Can do absolutely, however this means that in save_all I can't check if the
function needs to be called or not. For example, without CONFIG_VSX, MSR_VSX
won't exist which means we might end up calling save_vsx THEN checking MSR_VSX
and returning early.

I'm happy to defer to you and mpe on what's nicer, I would side with avoiding
the function call at the cost of ugly #ifdefs but I can always see the merits
of clean code.

Thanks for the review,

Cyril

Mikey

quoted

 #endif
 #ifdef CONFIG_SPE
 	if (usermsr & MSR_SPE)

diff --git a/arch/powerpc/kernel/vector.S

b/arch/powerpc/kernel/vector.S
index 51b0c17..1c2e7a3 100644

--- a/arch/powerpc/kernel/vector.S
+++ b/arch/powerpc/kernel/vector.S

@@ -151,23 +151,6 @@ _GLOBAL(load_up_vsx)
 	std	r12,_MSR(r1)
 	b	fast_exception_return
 
-/*
- * __giveup_vsx(tsk)
- * Disable VSX for the task given as the argument.
- * Does NOT save vsx registers.
- */
-_GLOBAL(__giveup_vsx)
-	addi	r3,r3,THREAD		/* want THREAD of

task */
-	ld	r5,PT_REGS(r3)
-	cmpdi	0,r5,0
-	beq	1f
-	ld	r4,_MSR-STACK_FRAME_OVERHEAD(r5)
-	lis	r3,MSR_VSX@h
-	andc	r4,r4,r3		/* disable VSX for
previous task */
-	std	r4,_MSR-STACK_FRAME_OVERHEAD(r5)
-1:
-	blr
-
 #endif /* CONFIG_VSX */