powerpc: Improve FSCR init and context switching

This fixes a few issues with FSCR init and switching.

In commit 152d523e6307 ("powerpc: Create context switch helpers
save_sprs() and restore_sprs()") we moved the setting of the FSCR
register from inside an CPU_FTR_ARCH_207S section to inside just a
CPU_FTR_ARCH_DSCR section. Hence we are setting FSCR on POWER6/7 where
the FSCR doesn't exist. This is harmless but we shouldn't do it.

Also, we can simplify the FSCR context switch. We don't need to go
through the calculation involving dscr_inherit. We can just restore
what we saved last time.

We also set an initial value in INIT_THREAD, so that pid 1 which is
cloned from that gets a sane value.

Based on patch by Jack Miller.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

3 files changed, 7 insertions, 9 deletions

diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index c0c27bdbb069..f6b1a5f51d05 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -347,6 +347,7 @@ struct thread_struct {
         .fs = KERNEL_DS, \
         .fpexc_mode = 0, \
         .ppr = INIT_PPR, \
+        .fscr = FSCR_TAR | FSCR_EBB \
 }
 #endif
 
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index c5c3ae2ef3c1..6d0a831bc7d8 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1031,18 +1031,11 @@ static inline void restore_sprs(struct thread_struct *old_thread,
 #ifdef CONFIG_PPC_BOOK3S_64
         if (cpu_has_feature(CPU_FTR_DSCR)) {
                 u64 dscr = get_paca()->dscr_default;
-                u64 fscr = old_thread->fscr & ~FSCR_DSCR;
-
-                if (new_thread->dscr_inherit) {
+                if (new_thread->dscr_inherit)
                         dscr = new_thread->dscr;
-                        fscr |= FSCR_DSCR;
-                }
 
                 if (old_thread->dscr != dscr)
                         mtspr(SPRN_DSCR, dscr);
-
-                if (old_thread->fscr != fscr)
-                        mtspr(SPRN_FSCR, fscr);
         }
 
         if (cpu_has_feature(CPU_FTR_ARCH_207S)) {
@@ -1053,6 +1046,9 @@ static inline void restore_sprs(struct thread_struct *old_thread,
                 if (old_thread->ebbrr != new_thread->ebbrr)
                         mtspr(SPRN_EBBRR, new_thread->ebbrr);
 
+                if (old_thread->fscr != new_thread->fscr)
+                        mtspr(SPRN_FSCR, new_thread->fscr);
+
                 if (old_thread->tar != new_thread->tar)
                         mtspr(SPRN_TAR, new_thread->tar);
         }
diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 11d15e7270e0..d2518c3cbf04 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -1419,7 +1419,8 @@ void facility_unavailable_exception(struct pt_regs *regs)
                         rd = (instword >> 21) & 0x1f;
                         current->thread.dscr = regs->gpr[rd];
                         current->thread.dscr_inherit = 1;
-                        mtspr(SPRN_FSCR, value | FSCR_DSCR);
+                        current->thread.fscr |= FSCR_DSCR;
+                        mtspr(SPRN_FSCR, current->thread.fscr);
                 }
 
                 /* Read from DSCR (mfspr RT, 0x03) */