[kernel] Save ring3 rflags in cpu_data, not just stack

So that kernel code can modify user rflags, save it in the CPU state
data, and save that off to the TCB when switching tasks.

src/kernel/cpu.h

@@ -28,6 +28,7 @@ struct cpu_data
     uint32_t reserved;
     uintptr_t rsp0;
     uintptr_t rsp3;
+    uint64_t rflags3;
     TCB *tcb;
     thread *thread;
     process *process;

src/kernel/objects/thread.h

@@ -15,6 +15,7 @@ struct TCB
     uintptr_t rsp;
     uintptr_t rsp0;
     uintptr_t rsp3;
+    uintptr_t rflags3;
     uintptr_t pml4;
 
     uint8_t priority;

src/kernel/syscall.s

@@ -31,6 +31,7 @@ syscall_handler_prelude:
 	swapgs
 	mov [gs:CPU_DATA.rsp3], rsp
 	mov rsp, [gs:CPU_DATA.rsp0]
+	mov [gs:CPU_DATA.rflags3], r11
 
 	push rcx
 	push rbp
@@ -44,7 +45,6 @@ syscall_handler_prelude:
 	mov rcx, r10
 
 	push rbx
-	push r11
 	push r12
 	push r13
 	push r14
@@ -76,12 +76,12 @@ kernel_to_user_trampoline:
 	pop r14
 	pop r13
 	pop r12
-	pop r11
 	pop rbx
 
 	pop rbp
 	pop rcx
 
+	mov r11, [gs:CPU_DATA.rflags3]
 	mov [gs:CPU_DATA.rsp0], rsp
 	mov rsp, [gs:CPU_DATA.rsp3]
 

src/kernel/task.s

@@ -20,6 +20,10 @@ task_switch:
 	mov rcx, [gs:CPU_DATA.rsp3]    ; rcx: current task's saved user rsp
 	mov [rax + TCB.rsp3], rcx
 
+	; Copy off saved user rflags
+	mov rcx, [gs:CPU_DATA.rflags3] ; rcx: current task's saved user rflags
+	mov [rax + TCB.rflags3], rcx
+
 	; Install next task's TCB
 	mov [gs:CPU_DATA.tcb], rdi     ; rdi: next TCB (function param)
 	mov rsp, [rdi + TCB.rsp]       ; next task's stack pointer
@@ -37,6 +41,10 @@ task_switch:
 	mov rcx, [rdi + TCB.rsp3]      ; rcx: new task's saved user rsp
 	mov [gs:CPU_DATA.rsp3], rcx
 
+	; Update saved user rflags
+	mov rcx, [rdi + TCB.rflags3]   ; rcx: new task's saved user rflags
+	mov [gs:CPU_DATA.rflags3], rcx
+
 	; check if we need to update CR3
 	mov rdx, cr3                   ; rdx: old CR3
 	cmp rax, rdx

src/kernel/tasking.inc

@@ -2,6 +2,7 @@ struc TCB
 .rsp:          resq 1
 .rsp0:         resq 1
 .rsp3:         resq 1
+.rflags3:      resq 1
 .pml4:         resq 1
 endstruc
 
@@ -12,6 +13,7 @@ struc CPU_DATA
 .reserved      resd 1
 .rsp0:         resq 1
 .rsp3:         resq 1
+.rflags3:      resq 1
 .tcb:          resq 1
 .thread:       resq 1
 .process:      resq 1