3.16.55-rc1 review patch. If anyone has any objections, please let me know.
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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); }