i387: Uninline the generic FP helpers that we expose to kernel modules

Instead of exporting the very low-level internals of the FPU state
save/restore code (ie things like 'fpu_owner_task'), we should export
the higher-level interfaces.

Inlining these things is pointless anyway: sure, sometimes the end
result is small, but while 'stts()' can result in just three x86
instructions, those are not cheap instructions (writing %cr0 is a
serializing instruction and a very slow one at that).

So the overhead of a function call is not noticeable, and we really
don't want random modules mucking about with our internal state save
logic anyway.

So this unexports 'fpu_owner_task', and instead uninlines and exports
the actual functions that modules can use: fpu_kernel_begin/end() and
unlazy_fpu().

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1202211339590.5354@i5.linux-foundation.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 80 insertions, 0 deletions

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 739d8598f78..17b7549c413 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -32,6 +32,86 @@
 # define user32_fxsr_struct     user_fxsr_struct
 #endif
 
+/*
+ * Were we in an interrupt that interrupted kernel mode?
+ *
+ * We can do a kernel_fpu_begin/end() pair *ONLY* if that
+ * pair does nothing at all: the thread must not have fpu (so
+ * that we don't try to save the FPU state), and TS must
+ * be set (so that the clts/stts pair does nothing that is
+ * visible in the interrupted kernel thread).
+ */
+static inline bool interrupted_kernel_fpu_idle(void)
+{
+        return !__thread_has_fpu(current) &&
+                (read_cr0() & X86_CR0_TS);
+}
+
+/*
+ * Were we in user mode (or vm86 mode) when we were
+ * interrupted?
+ *
+ * Doing kernel_fpu_begin/end() is ok if we are running
+ * in an interrupt context from user mode - we'll just
+ * save the FPU state as required.
+ */
+static inline bool interrupted_user_mode(void)
+{
+        struct pt_regs *regs = get_irq_regs();
+        return regs && user_mode_vm(regs);
+}
+
+/*
+ * Can we use the FPU in kernel mode with the
+ * whole "kernel_fpu_begin/end()" sequence?
+ *
+ * It's always ok in process context (ie "not interrupt")
+ * but it is sometimes ok even from an irq.
+ */
+bool irq_fpu_usable(void)
+{
+        return !in_interrupt() ||
+                interrupted_user_mode() ||
+                interrupted_kernel_fpu_idle();
+}
+EXPORT_SYMBOL(irq_fpu_usable);
+
+void kernel_fpu_begin(void)
+{
+        struct task_struct *me = current;
+
+        WARN_ON_ONCE(!irq_fpu_usable());
+        preempt_disable();
+        if (__thread_has_fpu(me)) {
+                __save_init_fpu(me);
+                __thread_clear_has_fpu(me);
+                /* We do 'stts()' in kernel_fpu_end() */
+        } else {
+                percpu_write(fpu_owner_task, NULL);
+                clts();
+        }
+}
+EXPORT_SYMBOL(kernel_fpu_begin);
+
+void kernel_fpu_end(void)
+{
+        stts();
+        preempt_enable();
+}
+EXPORT_SYMBOL(kernel_fpu_end);
+
+void unlazy_fpu(struct task_struct *tsk)
+{
+        preempt_disable();
+        if (__thread_has_fpu(tsk)) {
+                __save_init_fpu(tsk);
+                __thread_fpu_end(tsk);
+        } else
+                tsk->fpu_counter = 0;
+        preempt_enable();
+}
+EXPORT_SYMBOL(unlazy_fpu);
+
 #ifdef CONFIG_MATH_EMULATION
 # define HAVE_HWFP              (boot_cpu_data.hard_math)
 #else