x86/fpu: Synchronize the naming of drop_fpu() and fpu_reset_state()

drop_fpu() and fpu_reset_state() are similar in functionality
and in scope, yet this is not apparent from their names.

drop_fpu() deactivates FPU contents (both the fpregs and the fpstate),
but leaves register contents intact in the eager-FPU case, mostly as an
optimization. It disables fpregs in the lazy FPU case. The drop_fpu()
method can be used to destroy FPU state in an optimized way, when we
know that a new state will be loaded before user-space might see
any remains of the old FPU state:

     - such as in sys_exit()'s exit_thread() where we know this task
       won't execute any user-space instructions anymore and the
       next context switch cleans up the FPU. The old FPU state
       might still be around in the eagerfpu case but won't be
       saved.

     - in __restore_xstate_sig(), where we use drop_fpu() before
       copying a new state into the fpstate and activating that one.
       No user-pace instructions can execute between those steps.

     - in sys_execve()'s fpu__clear(): there we use drop_fpu() in
       the !eagerfpu case, where it's equivalent to a full reinit.

fpu_reset_state() is a stronger version of drop_fpu(): both in
the eagerfpu and the lazy-FPU case it guarantees that fpregs
are reinitialized to init state. This method is used in cases
where we need a full reset:

     - handle_signal() uses fpu_reset_state() to reset the FPU state
       to init before executing a user-space signal handler. While we
       have already saved the original FPU state at this point, and
       always restore the original state, the signal handling code
       still has to do this reinit, because signals may interrupt
       any user-space instruction, and the FPU might be in various
       intermediate states (such as an unbalanced x87 stack) that is
       not immediately usable for general C signal handler code.

     - __restore_xstate_sig() uses fpu_reset_state() when the signal
       frame has no FP context. Since the signal handler may have
       modified the FPU state, it gets reset back to init state.

     - in another branch __restore_xstate_sig() uses fpu_reset_state()
       to handle a restoration error: when restore_user_xstate() fails
       to restore FPU state and we might have inconsistent FPU data,
       fpu_reset_state() is used to reset it back to a known good
       state.

     - __kernel_fpu_end() uses fpu_reset_state() in an error branch.
       This is in a 'must not trigger' error branch, so on bug-free
       kernels this never triggers.

     - fpu__restore() uses fpu_reset_state() in an error path
       as well: if the fpstate was set up with invalid FPU state
       (via ptrace or via a signal handler), then it's reset back
       to init state.

     - likewise, the scheduler's switch_fpu_finish() uses it in a
       restoration error path too.

Move both drop_fpu() and fpu_reset_state() to the fpu__*() namespace
and harmonize their naming with their function:

    fpu__drop()
    fpu__reset()

This clearly shows that both methods operate on the full state of the
FPU, just like fpu__restore().

Also add comments to explain what each function does.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/fpu/internal.h

@@ -382,11 +382,17 @@ static inline void fpregs_deactivate(struct fpu *fpu)
 	__fpregs_deactivate_hw();
 }
 
-static inline void drop_fpu(struct fpu *fpu)
+/*
+ * Drops current FPU state: deactivates the fpregs and
+ * the fpstate. NOTE: it still leaves previous contents
+ * in the fpregs in the eager-FPU case.
+ *
+ * This function can be used in cases where we know that
+ * a state-restore is coming: either an explicit one,
+ * or a reschedule.
+ */
+static inline void fpu__drop(struct fpu *fpu)
 {
-	/*
-	 * Forget coprocessor state..
-	 */
 	preempt_disable();
 	fpu->counter = 0;
 
@@ -412,13 +418,12 @@ static inline void restore_init_xstate(void)
 }
 
 /*
- * Reset the FPU state in the eager case and drop it in the lazy case (later use
+ * Reset the FPU state back to init state.
- * will reinit it).
  */
-static inline void fpu_reset_state(struct fpu *fpu)
+static inline void fpu__reset(struct fpu *fpu)
 {
 	if (!use_eager_fpu())
-		drop_fpu(fpu);
+		fpu__drop(fpu);
 	else
 		restore_init_xstate();
 }
@@ -516,7 +521,7 @@ static inline void switch_fpu_finish(struct fpu *new_fpu, fpu_switch_t fpu_switc
 {
 	if (fpu_switch.preload) {
 		if (unlikely(restore_fpu_checking(new_fpu)))
-			fpu_reset_state(new_fpu);
+			fpu__reset(new_fpu);
 	}
 }
 

arch/x86/kernel/fpu/core.c

@@ -116,7 +116,7 @@ void __kernel_fpu_end(void)
 
 	if (fpu->fpregs_active) {
 		if (WARN_ON(restore_fpu_checking(fpu)))
-			fpu_reset_state(fpu);
+			fpu__reset(fpu);
 	} else {
 		__fpregs_deactivate_hw();
 	}
@@ -339,7 +339,7 @@ void fpu__restore(void)
 	kernel_fpu_disable();
 	fpregs_activate(fpu);
 	if (unlikely(restore_fpu_checking(fpu))) {
-		fpu_reset_state(fpu);
+		fpu__reset(fpu);
 		force_sig_info(SIGSEGV, SEND_SIG_PRIV, tsk);
 	} else {
 		tsk->thread.fpu.counter++;
@@ -360,7 +360,7 @@ void fpu__clear(struct task_struct *tsk)
 
 	if (!use_eager_fpu()) {
 		/* FPU state will be reallocated lazily at the first use. */
-		drop_fpu(fpu);
+		fpu__drop(fpu);
 	} else {
 		if (!fpu->fpstate_active) {
 			fpu__activate_curr(fpu);

arch/x86/kernel/fpu/xstate.c

@@ -401,7 +401,7 @@ int __restore_xstate_sig(void __user *buf, void __user *buf_fx, int size)
 			 config_enabled(CONFIG_IA32_EMULATION));
 
 	if (!buf) {
-		fpu_reset_state(fpu);
+		fpu__reset(fpu);
 		return 0;
 	}
 
@@ -449,7 +449,7 @@ int __restore_xstate_sig(void __user *buf, void __user *buf_fx, int size)
 		 * We will be ready to restore/save the state only after
 		 * fpu->fpstate_active is again set.
 		 */
-		drop_fpu(fpu);
+		fpu__drop(fpu);
 
 		if (__copy_from_user(&fpu->state.xsave, buf_fx, state_size) ||
 		    __copy_from_user(&env, buf, sizeof(env))) {
@@ -474,7 +474,7 @@ int __restore_xstate_sig(void __user *buf, void __user *buf_fx, int size)
 		 */
 		user_fpu_begin();
 		if (restore_user_xstate(buf_fx, xfeatures, fx_only)) {
-			fpu_reset_state(fpu);
+			fpu__reset(fpu);
 			return -1;
 		}
 	}

arch/x86/kernel/process.c

@@ -110,7 +110,7 @@ void exit_thread(void)
 		kfree(bp);
 	}
 
-	drop_fpu(fpu);
+	fpu__drop(fpu);
 }
 
 void flush_thread(void)

arch/x86/kernel/signal.c

@@ -667,7 +667,7 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 		 * Ensure the signal handler starts with the new fpu state.
 		 */
 		if (fpu->fpstate_active)
-			fpu_reset_state(fpu);
+			fpu__reset(fpu);
 	}
 	signal_setup_done(failed, ksig, stepping);
 }