[PATCH 3.16 093/254] MIPS: Clear [MSA]FPE CSR.Cause after notify_die()

Ben Hutchings
2018-02-28T16:15:29Z

From: James Hogan <james.hogan@imgtec.com>

commit 64bedffe496820dbb6b53302d80dd0f04db33d8e upstream.

When handling floating point exceptions (FPEs) and MSA FPEs the Cause
bits of the appropriate control and status register (FCSR for FPEs and
MSACSR for MSA FPEs) are read and cleared before enabling interrupts,
presumably so that it doesn't have to go through the pain of restoring
those bits if the process is pre-empted, since writing those bits would
cause another immediate exception while still in the kernel.

The bits aren't normally ever restored again, since userland never
expects to see them set.

However for virtualisation it is necessary for the kernel to be able to
restore these Cause bits, as the guest may have been interrupted in an
FP exception handler but before it could read the Cause bits. This can
be done by registering a die notifier, to get notified of the exception
when such a value is restored, and if the PC was at the instruction
which is used to restore the guest state, the handler can step over it
and continue execution. The Cause bits can then remain set without
causing further exceptions.

For this to work safely a few changes are made:
- __build_clear_fpe and __build_clear_msa_fpe no longer clear the Cause
bits, and now return from exception level with interrupts disabled
instead of enabled.
- do_fpe() now clears the Cause bits and enables interrupts after
notify_die() is called, so that the notifier can chose to return from
exception without this happening.
- do_msa_fpe() acts similarly, but now actually makes use of the second
argument (msacsr) and calls notify_die() with the new DIE_MSAFP,
allowing die notifiers to be informed of MSA FPEs too.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/mips/include/asm/kdebug.h | 3 ++-
arch/mips/kernel/genex.S | 14 ++++----------
arch/mips/kernel/traps.c | 16 +++++++++++++++-
3 files changed, 21 insertions(+), 12 deletions(-)

--- a/arch/mips/include/asm/kdebug.h
+++ b/arch/mips/include/asm/kdebug.h
@@ -10,7 +10,8 @@ enum die_val {
DIE_RI,
DIE_PAGE_FAULT,
DIE_BREAK,
- DIE_SSTEPBP
+ DIE_SSTEPBP,
+ DIE_MSAFP
};

#endif /* _ASM_MIPS_KDEBUG_H */
--- a/arch/mips/kernel/genex.S
+++ b/arch/mips/kernel/genex.S
@@ -360,21 +360,15 @@ NESTED(nmi_handler, PT_SIZE, sp)
.set mips1
SET_HARDFLOAT
cfc1 a1, fcr31
- li a2, ~(0x3f << 12)
- and a2, a1
- ctc1 a2, fcr31
.set pop
- TRACE_IRQS_ON
- STI
+ CLI
+ TRACE_IRQS_OFF
.endm

.macro __build_clear_msa_fpe
_cfcmsa a1, MSA_CSR
- li a2, ~(0x3f << 12)
- and a1, a1, a2
- _ctcmsa MSA_CSR, a1
- TRACE_IRQS_ON
- STI
+ CLI
+ TRACE_IRQS_OFF
.endm

.macro __build_clear_ade
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -744,6 +744,11 @@ asmlinkage void do_fpe(struct pt_regs *r
if (notify_die(DIE_FP, "FP exception", regs, 0, regs_to_trapnr(regs),
SIGFPE) == NOTIFY_STOP)
goto out;
+
+ /* Clear FCSR.Cause before enabling interrupts */
+ write_32bit_cp1_register(CP1_STATUS, fcr31 & ~FPU_CSR_ALL_X);
+ local_irq_enable();
+
die_if_kernel("FP exception in kernel code", regs);

if (fcr31 & FPU_CSR_UNI_X) {
@@ -1295,13 +1300,22 @@ out:
exception_exit(prev_state);
}

-asmlinkage void do_msa_fpe(struct pt_regs *regs)
+asmlinkage void do_msa_fpe(struct pt_regs *regs, unsigned int msacsr)
{
enum ctx_state prev_state;

prev_state = exception_enter();
+ if (notify_die(DIE_MSAFP, "MSA FP exception", regs, 0,
+ regs_to_trapnr(regs), SIGFPE) == NOTIFY_STOP)
+ goto out;
+
+ /* Clear MSACSR.Cause before enabling interrupts */
+ write_msa_csr(msacsr & ~MSA_CSR_CAUSEF);
+ local_irq_enable();
+
die_if_kernel("do_msa_fpe invoked from kernel context!", regs);
force_sig(SIGFPE, current);
+out:
exception_exit(prev_state);
}