powerpc/64s: Fix restore of NV GPRs after facility unavailable exception

Commit b58ba04500c7 ("powerpc/64s/exception: Remove lite interrupt
return") changed the interrupt return path to not restore non-volatile
registers by default, and explicitly restore them in paths where it is
required.

But it missed that the facility unavailable exception can sometimes
modify user registers, ie. when it does emulation of move from DSCR.

This is seen as a failure of the dscr_sysfs_thread_test:
  test: dscr_sysfs_thread_test
  [cpu 0] User DSCR should be 1 but is 0
  failure: dscr_sysfs_thread_test

So restore non-volatile GPRs after facility unavailable exceptions.

Currently the hypervisor facility unavailable exception is also wired
up to call facility_unavailable_exception().

In practice we should never take a hypervisor facility unavailable
exception for the DSCR. On older bare metal systems we set HFSCR_DSCR
unconditionally in __init_HFSCR, or on newer systems it should be
enabled via the "data-stream-control-register" device tree CPU
feature.

Even if it's not, since commit b2239b36278f ("KVM: PPC: Book3S HV:
Don't access HFSCR, LPIDR or LPCR when running nested"), the KVM code
has unconditionally set HFSCR_DSCR when running guests.

So we should only get a hypervisor facility unavailable for the DSCR
if skiboot has disabled the "data-stream-control-register" feature,
and we are somehow in guest context but not via KVM.

Given all that, it should be unnecessary to add a restore of
non-volatile GPRs after the hypervisor facility exception, because we
never expect to hit that path. But equally we may as well add the
restore, because we never expect to hit that path, and if we ever did,
at least we would correctly restore the registers to their post
emulation state.

In future we can split the non-HV and HV facility unavailable handling
so that there is no emulation in the HV handler, and then remove the
restore for the HV case.

Fixes: b58ba04500c7 ("powerpc/64s/exception: Remove lite interrupt return")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200526061808.2472279-1-mpe@ellerman.id.au

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index b0ad930cbae5caa2b39f581dea5e999740a95e00..ebeebab74b564b7960a5270f180721d430d04dd0 100644 (file)
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -2411,6 +2411,7 @@ EXC_COMMON_BEGIN(facility_unavailable_common)
        GEN_COMMON facility_unavailable
        addi    r3,r1,STACK_FRAME_OVERHEAD
        bl      facility_unavailable_exception
+       REST_NVGPRS(r1) /* instruction emulation may change GPRs */
        b       interrupt_return
 
        GEN_KVM facility_unavailable
@@ -2440,6 +2441,7 @@ EXC_COMMON_BEGIN(h_facility_unavailable_common)
        GEN_COMMON h_facility_unavailable
        addi    r3,r1,STACK_FRAME_OVERHEAD
        bl      facility_unavailable_exception
+       REST_NVGPRS(r1) /* XXX Shouldn't be necessary in practice */
        b       interrupt_return
 
        GEN_KVM h_facility_unavailable