From: Michael Ellerman mpe@ellerman.id.au
commit 0828137e8f16721842468e33df0460044a0c588b upstream.
__init_FSCR() was added originally in commit 2468dcf641e4 ("powerpc: Add support for context switching the TAR register") (Feb 2013), and only set FSCR_TAR.
At that point FSCR (Facility Status and Control Register) was not context switched, so the setting was permanent after boot.
Later we added initialisation of FSCR_DSCR to __init_FSCR(), in commit 54c9b2253d34 ("powerpc: Set DSCR bit in FSCR setup") (Mar 2013), again that was permanent after boot.
Then commit 2517617e0de6 ("powerpc: Fix context switch DSCR on POWER8") (Aug 2013) added a limited context switch of FSCR, just the FSCR_DSCR bit was context switched based on thread.dscr_inherit. That commit said "This clears the H/FSCR DSCR bit initially", but it didn't, it left the initialisation of FSCR_DSCR in __init_FSCR(). However the initial context switch from init_task to pid 1 would clear FSCR_DSCR because thread.dscr_inherit was 0.
That commit also introduced the requirement that FSCR_DSCR be clear for user processes, so that we can take the facility unavailable interrupt in order to manage dscr_inherit.
Then in commit 152d523e6307 ("powerpc: Create context switch helpers save_sprs() and restore_sprs()") (Dec 2015) FSCR was added to thread_struct. However it still wasn't fully context switched, we just took the existing value and set FSCR_DSCR if the new thread had dscr_inherit set. FSCR was still initialised at boot to FSCR_DSCR | FSCR_TAR, but that value was not propagated into the thread_struct, so the initial context switch set FSCR_DSCR back to 0.
Finally commit b57bd2de8c6c ("powerpc: Improve FSCR init and context switching") (Jun 2016) added a full context switch of the FSCR, and added an initialisation of init_task.thread.fscr to FSCR_TAR | FSCR_EBB, but omitted FSCR_DSCR.
The end result is that swapper runs with FSCR_DSCR set because of the initialisation in __init_FSCR(), but no other processes do, they use the value from init_task.thread.fscr.
Having FSCR_DSCR set for swapper allows it to access SPR 3 from userspace, but swapper never runs userspace, so it has no useful effect. It's also confusing to have the value initialised in two places to two different values.
So remove FSCR_DSCR from __init_FSCR(), this at least gets us to the point where there's a single value of FSCR, even if it's still set in two places.
Signed-off-by: Michael Ellerman mpe@ellerman.id.au Tested-by: Alistair Popple alistair@popple.id.au Link: https://lore.kernel.org/r/20200527145843.2761782-1-mpe@ellerman.id.au Cc: Thadeu Lima de Souza Cascardo cascardo@canonical.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/powerpc/kernel/cpu_setup_power.S | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/arch/powerpc/kernel/cpu_setup_power.S +++ b/arch/powerpc/kernel/cpu_setup_power.S @@ -183,7 +183,7 @@ __init_LPCR_ISA300:
__init_FSCR: mfspr r3,SPRN_FSCR - ori r3,r3,FSCR_TAR|FSCR_DSCR|FSCR_EBB + ori r3,r3,FSCR_TAR|FSCR_EBB mtspr SPRN_FSCR,r3 blr