s390/kernel: lazy restore fpu registers

Improve the save and restore behavior of FPU register contents to use the
vector extension within the kernel.

The kernel does not use floating-point or vector registers and, therefore,
saving and restoring the FPU register contents are performed for handling
signals or switching processes only.  To prepare for using vector
instructions and vector registers within the kernel, enhance the save
behavior and implement a lazy restore at return to user space from a
system call or interrupt.

To implement the lazy restore, the save_fpu_regs() sets a CPU information
flag, CIF_FPU, to indicate that the FPU registers must be restored.
Saving and setting CIF_FPU is performed in an atomic fashion to be
interrupt-safe.  When the kernel wants to use the vector extension or
wants to change the FPU register state for a task during signal handling,
the save_fpu_regs() must be called first.  The CIF_FPU flag is also set at
process switch.  At return to user space, the FPU state is restored.  In
particular, the FPU state includes the floating-point or vector register
contents, as well as, vector-enablement and floating-point control.  The
FPU state restore and clearing CIF_FPU is also performed in an atomic
fashion.

For KVM, the restore of the FPU register state is performed when restoring
the general-purpose guest registers before the SIE instructions is started.
Because the path towards the SIE instruction is interruptible, the CIF_FPU
flag must be checked again right before going into SIE.  If set, the guest
registers must be reloaded again by re-entering the outer SIE loop.  This
is the same behavior as if the SIE critical section is interrupted.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 26 insertions, 7 deletions

diff --git a/arch/s390/kernel/process.c b/arch/s390/kernel/process.c
index 56949c9cda97..9cf0063f920e 100644
--- a/arch/s390/kernel/process.c
+++ b/arch/s390/kernel/process.c
@@ -90,16 +90,28 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 	*dst = *src;
 
 	/* Set up a new floating-point register save area */
+	dst->thread.fpu.fpc = 0;
+	dst->thread.fpu.flags = 0;	/* Always start with VX disabled */
 	dst->thread.fpu.fprs = kzalloc(sizeof(freg_t) * __NUM_FPRS,
 				       GFP_KERNEL|__GFP_REPEAT);
 	if (!dst->thread.fpu.fprs)
 		return -ENOMEM;
 
-	/* Save the fpu registers to new thread structure. */
-	save_fp_ctl(&dst->thread.fpu.fpc);
-	save_fp_regs(dst->thread.fpu.fprs);
-	dst->thread.fpu.flags = 0;     /* Always start with VX disabled */
-
+	/*
+	 * Save the floating-point or vector register state of the current
+	 * task.  The state is not saved for early kernel threads, for example,
+	 * the init_task, which do not have an allocated save area.
+	 * The CIF_FPU flag is set in any case to lazy clear or restore a saved
+	 * state when switching to a different task or returning to user space.
+	 */
+	save_fpu_regs(&current->thread.fpu);
+	dst->thread.fpu.fpc = current->thread.fpu.fpc;
+	if (is_vx_task(current))
+		convert_vx_to_fp(dst->thread.fpu.fprs,
+				 current->thread.fpu.vxrs);
+	else
+		memcpy(dst->thread.fpu.fprs, current->thread.fpu.fprs,
+		       sizeof(freg_t) * __NUM_FPRS);
 	return 0;
 }
 
@@ -184,8 +196,15 @@ asmlinkage void execve_tail(void)
  */
 int dump_fpu (struct pt_regs * regs, s390_fp_regs *fpregs)
 {
-	save_fp_ctl(&fpregs->fpc);
-	save_fp_regs(fpregs->fprs);
+	save_fpu_regs(&current->thread.fpu);
+	fpregs->fpc = current->thread.fpu.fpc;
+	fpregs->pad = 0;
+	if (is_vx_task(current))
+		convert_vx_to_fp((freg_t *)&fpregs->fprs,
+				 current->thread.fpu.vxrs);
+	else
+		memcpy(&fpregs->fprs, current->thread.fpu.fprs,
+		       sizeof(fpregs->fprs));
 	return 1;
 }
 EXPORT_SYMBOL(dump_fpu);