From: David van Moolenbroek <david@minix3.org>
Date: Sat, 3 Mar 2012 16:48:14 +0000 (+0100)
Subject: Kernel: retain FPU state upon save
X-Git-Url: http://zhaoyanbai.com/repos/?a=commitdiff_plain;h=ffa4d7423ab418a96654f22fcaec0da220060399;p=minix.git

Kernel: retain FPU state upon save

On the x86, saving FPU state has the side effect of resetting this
state. In some cases (fork, getcontext), this would cause the state
to be lost. This patch restores the FPU state right after saving it,
except when different state is loaded immediately after.
---

diff --git a/kernel/arch/i386/arch_system.c b/kernel/arch/i386/arch_system.c
index 505b50f53..be8503a57 100644
--- a/kernel/arch/i386/arch_system.c
+++ b/kernel/arch/i386/arch_system.c
@@ -290,20 +290,16 @@ PUBLIC void save_local_fpu(struct proc *pr)
 
 PUBLIC void save_fpu(struct proc *pr)
 {
+	int r;
+
 #ifdef CONFIG_SMP
-	if (cpuid == pr->p_cpu) {
-		if (get_cpulocal_var(fpu_owner) == pr) {
-			disable_fpu_exception();
-			save_local_fpu(pr);
-		}
-	}
-	else {
+	if (cpuid != pr->p_cpu) {
 		int stopped;
 
 		/* remember if the process was already stopped */
 		stopped = RTS_ISSET(pr, RTS_PROC_STOP);
 
-		/* stop the remote process and force it's context to be saved */
+		/* stop the remote process and force its context to be saved */
 		smp_schedule_stop_proc_save_ctx(pr);
 
 		/*
@@ -313,13 +309,21 @@ PUBLIC void save_fpu(struct proc *pr)
 		 */
 		if (!stopped)
 			RTS_UNSET(pr, RTS_PROC_STOP);
+
+		return;
 	}
-#else
+#endif
+
 	if (get_cpulocal_var(fpu_owner) == pr) {
 		disable_fpu_exception();
 		save_local_fpu(pr);
+
+		/* The state may now be reset, and the caller does not expect
+		 * this. Immediately restore the saved state.
+		 */
+		r = restore_fpu(pr);
+		assert(r == OK);
 	}
-#endif
 }
 
 PUBLIC int restore_fpu(struct proc *pr)
diff --git a/test/test51.c b/test/test51.c
index e896c6da2..0a8d83140 100644
--- a/test/test51.c
+++ b/test/test51.c
@@ -106,6 +106,7 @@ void do_child(void)
 
 void do_parent(void)
 {
+  ucontext_t dummy;
   int s;
   s = 1;
 
@@ -116,11 +117,16 @@ void do_parent(void)
      between context swaps. */
   if (fesetround(FE_UPWARD) != 0) err(10, 2);
 
+  /* Quick check to make sure that getcontext does not reset the FPU state. */
+  getcontext(&dummy);
+
+  if (fegetround() != FE_UPWARD) err(10, 3);
+
   while(s < SWAPS) {
 	do_calcs();
-  	if (fegetround() != FE_UPWARD) err(10, 3);
+	if (fegetround() != FE_UPWARD) err(10, 4);
 	s++;
-	if (swapcontext(&ctx[1], &ctx[2]) == -1) err(10, 4);
+	if (swapcontext(&ctx[1], &ctx[2]) == -1) err(10, 5);
   }
   /* Returning to main thread through uc_link */
 }