powerpc: Fix regression of per-CPU DSCR setting

Since commit "efcac658 powerpc: Per process DSCR + some fixes (try#4)"
it is no longer possible to set the DSCR on a per-CPU basis.

The old behaviour was to minipulate the DSCR SPR directly but this is no
longer sufficient: the value is quickly overwritten by context switching.

This patch stores the per-CPU DSCR value in a kernel variable rather than
directly in the SPR and it is used whenever a process has not set the DSCR
itself. The sysfs interface (/sys/devices/system/cpu/cpuN/dscr) is unchanged.

Writes to the old global default (/sys/devices/system/cpu/dscr_default)
now set all of the per-CPU values and reads return the last written value.

The new per-CPU default is added to the paca_struct and is used everywhere
outside of sysfs.c instead of the old global default.
Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/paca.h

@@ -92,7 +92,10 @@ struct paca_struct {
 	struct slb_shadow *slb_shadow_ptr;
 	struct dtl_entry *dispatch_log;
 	struct dtl_entry *dispatch_log_end;
+#endif /* CONFIG_PPC_STD_MMU_64 */
+	u64 dscr_default;		/* per-CPU default DSCR */
 
+#ifdef CONFIG_PPC_STD_MMU_64
 	/*
 	 * Now, starting in cacheline 2, the exception save areas
 	 */

arch/powerpc/kernel/asm-offsets.c

@@ -247,6 +247,7 @@ int main(void)
 #endif
 	DEFINE(PACAHWCPUID, offsetof(struct paca_struct, hw_cpu_id));
 	DEFINE(PACAKEXECSTATE, offsetof(struct paca_struct, kexec_state));
+	DEFINE(PACA_DSCR, offsetof(struct paca_struct, dscr_default));
 	DEFINE(PACA_STARTTIME, offsetof(struct paca_struct, starttime));
 	DEFINE(PACA_STARTTIME_USER, offsetof(struct paca_struct, starttime_user));
 	DEFINE(PACA_USER_TIME, offsetof(struct paca_struct, user_time));

arch/powerpc/kernel/entry_64.S

@@ -387,12 +387,6 @@ _GLOBAL(ret_from_kernel_thread)
 	li	r3,0
 	b	syscall_exit
 
-	.section	".toc","aw"
-DSCR_DEFAULT:
-	.tc dscr_default[TC],dscr_default
-
-	.section	".text"
-
 /*
  * This routine switches between two different tasks.  The process
  * state of one is saved on its kernel stack.  Then the state
@@ -577,11 +571,10 @@ END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
 #ifdef CONFIG_PPC64
 BEGIN_FTR_SECTION
 	lwz	r6,THREAD_DSCR_INHERIT(r4)
-	ld	r7,DSCR_DEFAULT@toc(2)
 	ld	r0,THREAD_DSCR(r4)
 	cmpwi	r6,0
 	bne	1f
-	ld	r0,0(r7)
+	ld	r0,PACA_DSCR(r13)
 1:
 BEGIN_FTR_SECTION_NESTED(70)
 	mfspr	r8, SPRN_FSCR

arch/powerpc/kernel/sysfs.c

@@ -484,7 +484,6 @@ SYSFS_PMCSETUP(pmc8, SPRN_PMC8);
 SYSFS_PMCSETUP(mmcra, SPRN_MMCRA);
 SYSFS_SPRSETUP(purr, SPRN_PURR);
 SYSFS_SPRSETUP(spurr, SPRN_SPURR);
-SYSFS_SPRSETUP(dscr, SPRN_DSCR);
 SYSFS_SPRSETUP(pir, SPRN_PIR);
 
 /*
@@ -494,12 +493,27 @@ SYSFS_SPRSETUP(pir, SPRN_PIR);
 */
 static DEVICE_ATTR(mmcra, 0600, show_mmcra, store_mmcra);
 static DEVICE_ATTR(spurr, 0400, show_spurr, NULL);
-static DEVICE_ATTR(dscr, 0600, show_dscr, store_dscr);
 static DEVICE_ATTR(purr, 0400, show_purr, store_purr);
 static DEVICE_ATTR(pir, 0400, show_pir, NULL);
 
-unsigned long dscr_default = 0;
-EXPORT_SYMBOL(dscr_default);
+static unsigned long dscr_default;
+
+static void read_dscr(void *val)
+{
+	*(unsigned long *)val = get_paca()->dscr_default;
+}
+
+static void write_dscr(void *val)
+{
+	get_paca()->dscr_default = *(unsigned long *)val;
+	if (!current->thread.dscr_inherit) {
+		current->thread.dscr = *(unsigned long *)val;
+		mtspr(SPRN_DSCR, *(unsigned long *)val);
+	}
+}
+
+SYSFS_SPRSETUP_SHOW_STORE(dscr);
+static DEVICE_ATTR(dscr, 0600, show_dscr, store_dscr);
 
 static void add_write_permission_dev_attr(struct device_attribute *attr)
 {
@@ -512,14 +526,6 @@ static ssize_t show_dscr_default(struct device *dev,
 	return sprintf(buf, "%lx\n", dscr_default);
 }
 
-static void update_dscr(void *dummy)
-{
-	if (!current->thread.dscr_inherit) {
-		current->thread.dscr = dscr_default;
-		mtspr(SPRN_DSCR, dscr_default);
-	}
-}
-
 static ssize_t __used store_dscr_default(struct device *dev,
 		struct device_attribute *attr, const char *buf,
 		size_t count)
@@ -532,7 +538,7 @@ static ssize_t __used store_dscr_default(struct device *dev,
 		return -EINVAL;
 	dscr_default = val;
 
-	on_each_cpu(update_dscr, NULL, 1);
+	on_each_cpu(write_dscr, &val, 1);
 
 	return count;
 }

arch/powerpc/kernel/tm.S

@@ -78,12 +78,6 @@ _GLOBAL(tm_abort)
 	TABORT(R3)
 	blr
 
-	.section	".toc","aw"
-DSCR_DEFAULT:
-	.tc dscr_default[TC],dscr_default
-
-	.section	".text"
-
 /* void tm_reclaim(struct thread_struct *thread,
  *                 unsigned long orig_msr,
  *		   uint8_t cause)
@@ -298,9 +292,8 @@ dont_backup_fp:
 	mtlr	r0
 	ld	r2, STK_GOT(r1)
 
-	/* Load system default DSCR */
-	ld	r4, DSCR_DEFAULT@toc(r2)
-	ld	r0, 0(r4)
+	/* Load CPU's default DSCR */
+	ld	r0, PACA_DSCR(r13)
 	mtspr	SPRN_DSCR, r0
 
 	blr
@@ -479,9 +472,8 @@ restore_gprs:
 	mtlr	r0
 	ld	r2, STK_GOT(r1)
 
-	/* Load system default DSCR */
-	ld	r4, DSCR_DEFAULT@toc(r2)
-	ld	r0, 0(r4)
+	/* Load CPU's default DSCR */
+	ld	r0, PACA_DSCR(r13)
 	mtspr	SPRN_DSCR, r0
 
 	blr

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -286,8 +286,7 @@ kvm_start_guest:
 	beq	kvm_no_guest
 
 	/* Set HSTATE_DSCR(r13) to something sensible */
-	LOAD_REG_ADDR(r6, dscr_default)
-	ld	r6, 0(r6)
+	ld	r6, PACA_DSCR(r13)
 	std	r6, HSTATE_DSCR(r13)
 
 	bl	kvmppc_hv_entry