x86-64: properly set up x87 state

kernel-src/arch/x86-64/cpu.c

@@ -40,4 +40,13 @@ void cpu_initstate() {
 		"or  $0b11000000000, %%rax;"
 		"mov %%rax, %%cr4;"
 		: : : "rax");
+
+	// set NE in cr0 and reset x87 fpu
+	asm volatile(
+		"fninit;"
+		"mov %%cr0, %%rax;"
+		"or $0b100000, %%rax;"
+		"mov %%rax, %%cr0;"
+		: : : "rax"
+	);
 }

kernel-src/include/x86-64/arch/context.h

@@ -54,8 +54,14 @@ typedef struct {
 	} \
 	(x)->rflags = 0x200;
 
-#define CTX_XINIT(x, u) \
-	(x)->mxcsr = 0x1F80; // all exceptions masked
+// all sse exceptions masked
+// initialise the x87 FPU state as it would be after the FNINIT instruction
+#define CTX_XINIT(x, u) {\
+	(x)->mxcsr = 0x1F80; \
+	*((uint16_t *)&(x)->fx[0]) = 0x37f; \
+	*((uint16_t *)&(x)->fx[2]) = 0; \
+	(x)->fx[4] = 0; \
+	}
 
 #define CTX_SP(x) (x)->rsp
 #define CTX_IP(x) (x)->rip