MIPS,prctl: add PR_[GS]ET_FP_MODE prctl options for MIPS

Userland code may be built using an ABI which permits linking to objects
that have more restrictive floating point requirements. For example,
userland code may be built to target the O32 FPXX ABI. Such code may be
linked with other FPXX code, or code built for either one of the more
restrictive FP32 or FP64. When linking with more restrictive code, the
overall requirement of the process becomes that of the more restrictive
code. The kernel has no way to know in advance which mode the process
will need to be executed in, and indeed it may need to change during
execution. The dynamic loader is the only code which will know the
overall required mode, and so it needs to have a means to instruct the
kernel to switch the FP mode of the process.

This patch introduces 2 new options to the prctl syscall which provide
such a capability. The FP mode of the process is represented as a
simple bitmask combining a number of mode bits mirroring those present
in the hardware. Userland can either retrieve the current FP mode of
the process:

  mode = prctl(PR_GET_FP_MODE);

or modify the current FP mode of the process:

  err = prctl(PR_SET_FP_MODE, new_mode);

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8899/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index ad3d2031c327..d5fbfb51b9da 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -1134,10 +1134,29 @@ static int default_cu2_call(struct notifier_block *nfb, unsigned long action,
 	return NOTIFY_OK;
 }
 
+static int wait_on_fp_mode_switch(atomic_t *p)
+{
+	/*
+	 * The FP mode for this task is currently being switched. That may
+	 * involve modifications to the format of this tasks FP context which
+	 * make it unsafe to proceed with execution for the moment. Instead,
+	 * schedule some other task.
+	 */
+	schedule();
+	return 0;
+}
+
 static int enable_restore_fp_context(int msa)
 {
 	int err, was_fpu_owner, prior_msa;
 
+	/*
+	 * If an FP mode switch is currently underway, wait for it to
+	 * complete before proceeding.
+	 */
+	wait_on_atomic_t(&current->mm->context.fp_mode_switching,
+			 wait_on_fp_mode_switch, TASK_KILLABLE);
+
 	if (!used_math()) {
 		/* First time FP context user. */
 		preempt_disable();