powerpc: fix regression of per-CPU DSCR setting

Hello,

This patch corrects a regression on PowerPC CPUs that causes their per-CPU DSCR SPR value (exposed via /sys/devices/system/cpuN/dscr) to be quickly lost during context switching, effectively meaning that the DSCR can no longer be set on a per-CPU basis.

My intent is to restore the functionality of the per-CPU value in a way that is compatible with the newer global default and task-specific DSCR setting system. Users of either the old or new systems should now get pretty much what they expect.

A couple of notes:

I've split an existing "ifdef CONFIG_PPC_STD_MMU_64" block in paca_struct into two parts because it allows dscr_default to be placed into a cache line hole. (This seems be the case even without CONFIG_PPC_STD_MMU_64 being defined.) Comments or ideas on alternative placements are welcome.

PowerPC context switching is touched but there should not be any performance cost; if anything it should get slightly faster due to the per-CPU value being easier to access than the old global default.

Sam Bobroff (3):
powerpc: Split __SYSFS_SPRSETUP macro
powerpc: fix regression of per-CPU DSCR setting
powerpc: Document sysfs DSCR interface

 Documentation/ABI/stable/sysfs-devices-system-cpu |   25 ++++++++++
 arch/powerpc/include/asm/paca.h                   |    3 ++
 arch/powerpc/kernel/asm-offsets.c                 |    1 +
 arch/powerpc/kernel/entry_64.S                    |    9 +---
 arch/powerpc/kernel/sysfs.c                       |   51 +++++++++++++--------
 arch/powerpc/kernel/tm.S                          |   16 ++-----
 arch/powerpc/kvm/book3s_hv_rmhandlers.S           |    3 +-
 7 files changed, 67 insertions(+), 41 deletions(-)
 create mode 100644 Documentation/ABI/stable/sysfs-devices-system-cpu

1.7.10.4