On Fri, 2020-05-29 at 11:24 +1000, Alistair Popple wrote: > For what it's worth I tested this series on Mambo PowerNV and it seems to > correctly enable/disable the prefix FSCR bit based on the cpu feature so feel > free to add: > > Tested-by: Alistair Popple <alist...@popple.id.au> > > Mikey is going to test out pseries.
FWIW this worked for me in the P10 + powervm sim testing. Tested-by: Michael Neuling <mi...@neuling.org> > > - Alistair > > On Thursday, 28 May 2020 12:58:40 AM AEST Michael Ellerman wrote: > > __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 <m...@ellerman.id.au> > > --- > > arch/powerpc/kernel/cpu_setup_power.S | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/arch/powerpc/kernel/cpu_setup_power.S > > b/arch/powerpc/kernel/cpu_setup_power.S index a460298c7ddb..f91ecb10d0ae > > 100644 > > --- a/arch/powerpc/kernel/cpu_setup_power.S > > +++ b/arch/powerpc/kernel/cpu_setup_power.S > > @@ -184,7 +184,7 @@ _GLOBAL(__restore_cpu_power9) > > > > __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 > >