kernel/x86_64: Reset FPU state in x86_64_thread_entry.

Otherwise it won't be reset at all and we will begin with the
previous thread's FPU state, both for userland and kernel threads.

Also clear the FPU state in x86_return_to_userland, just like
in syscall exit.

Change-Id: Ie46d0e64a680c860c7fbff8dc57116625724eadd
Reviewed-on: https://review.haiku-os.org/c/haiku/+/9092
Tested-by: Commit checker robot <[email protected]>
Haiku-Format: Haiku-format Bot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

src/system/kernel/arch/x86/64/arch.S

@@ -13,6 +13,8 @@
 
 #include <asm_defs.h>
 
+#include <arch/x86/arch_altcodepatch.h>
+
 #include "asm_offsets.h"
 #include "syscall_numbers.h"
 
@@ -29,6 +31,17 @@ FUNCTION_END(x86_get_stack_frame)
 
 /* void x86_64_thread_entry(); */
 FUNCTION(x86_64_thread_entry):
+	// Get thread pointer.
+	pop		%r12
+
+	// Reset FPU state.
+	movl	(gXsaveMask), %eax
+	movl	(gXsaveMask+4), %edx
+	leaq	THREAD_user_fpu_state(%r12), %rdi
+	CODEPATCH_START
+	fxrstorq	(%rdi)
+	CODEPATCH_END(ALTCODEPATCH_TAG_XRSTOR)
+
 	xorq	%rbp, %rbp
 
 	movq	%rsp, %rax

src/system/kernel/arch/x86/64/interrupts.S

@@ -626,9 +626,10 @@ FUNCTION(x86_return_to_userland):
 			, THREAD_flags(%r12)
 	jnz		.Luserland_return_work
 
-	// update the thread's kernel time and return
 	UPDATE_THREAD_KERNEL_TIME()
 
+	CLEAR_FPU_STATE()
+
 	// Restore the frame and return.
 	RESTORE_IFRAME()
 	swapgs
@@ -651,10 +652,10 @@ FUNCTION(x86_return_to_userland):
 	movq	%rbp, %rdi
 	call	x86_init_user_debug_at_kernel_exit
 1:
-	// Restore the saved registers.
+	CLEAR_FPU_STATE()
-	RESTORE_IFRAME()
 
-	// Restore the previous GS base and return.
+	// Restore the frame and return.
+	RESTORE_IFRAME()
 	swapgs
 	iretq
 .Luserland_return_handle_signals:

src/system/kernel/arch/x86/64/thread.cpp

@@ -251,6 +251,7 @@ arch_thread_init_kthread_stack(Thread* thread, void* _stack, void* _stackTop,
 
 	*--stackTop = uintptr_t(data);
 	*--stackTop = uintptr_t(function);
+	*--stackTop = uintptr_t(thread);
 
 	// Save the stack position.
 	thread->arch_info.current_stack = stackTop;