hw: enable FPU during CPU startup on x86

Also disable TS (task switch) flag in cr0 during kernel initialization,
so FPU faults are not raised. This became necessary since GCC lately
aggressively generates FPU instructions at arbitrary places and also at
early kernel-bootstrapping stages.

fixes #3365

repos/base-hw/src/bootstrap/spec/x86_64/crt0.s

@@ -157,6 +157,23 @@ __gdt:
 	movq (%rax), %rsp
 	addq %rcx, %rsp
 
+	/*
+	 * Enable paging and FPU:
+	 * PE, MP, NE, WP, PG
+	 */
+	mov $0x80010023, %rax
+	mov %rax, %cr0
+
+	/*
+	 * OSFXSR and OSXMMEXCPT for SSE FPU support
+	 */
+	mov %cr4, %rax
+	bts $9,   %rax
+	bts $10,  %rax
+	mov %rax, %cr4
+
+	fninit
+
 	/* kernel-initialization */
 	call init
 

repos/base-hw/src/core/spec/x86_64/fpu.cc

@@ -21,7 +21,12 @@ void Genode::Fpu::init()
 
 	Cpu::Cr0::Mp::set(cr0_value);
 	Cpu::Cr0::Em::clear(cr0_value);
-	Cpu::Cr0::Ts::set(cr0_value);
+
+	/*
+	 * Clear task switched so we do not gnerate FPU faults during kernel
+	 * initialisation, it will be turned on by Fpu::disable
+	 */
+	Cpu::Cr0::Ts::clear(cr0_value);
 	Cpu::Cr0::Ne::set(cr0_value);
 	Cpu::Cr0::write(cr0_value);