From 7b91cb3f96d7b8ba0874a1b24b19f624e43c99d5 Mon Sep 17 00:00:00 2001
From: Nathan Wagner <nw@hydaspes.if.org>
Date: Mon, 24 Oct 2016 20:35:17 -0500
Subject: [PATCH] syscalls and a user mode process

---
 syscall.h  |   3 ++
 syscalls.s | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 128 insertions(+)
 create mode 100644 syscall.h
 create mode 100644 syscalls.s

diff --git a/syscall.h b/syscall.h
new file mode 100644
index 0000000..04aebb6
--- /dev/null
+++ b/syscall.h
@@ -0,0 +1,3 @@
+void ksyscall(int call);
+void syscall_entry();
+void init_syscalls(void);
diff --git a/syscalls.s b/syscalls.s
new file mode 100644
index 0000000..2dd2f6a
--- /dev/null
+++ b/syscalls.s
@@ -0,0 +1,125 @@
+global syscall_entry:function
+global ksyscall:function
+
+extern printk
+extern allstop
+extern need_schedule
+
+; syscall to kernel
+; syscall number in rax
+; fourth argument (rcx) on stack?
+ksyscall:
+	; syscall over-writes rcx, save it
+	push rcx
+	syscall
+	pop rcx
+	ret
+
+%define STAR 0xC0000081
+%define LSTAR 0xC0000082
+%define FMASK 0xC0000084
+
+extern enable_syscall
+
+global init_syscalls:function
+init_syscalls:
+	; STAR register
+	mov ecx, STAR
+	rdmsr
+	mov edx, 0x23 ; user code segment / user data segment / 64 bit user code segment
+	shr edx, 16
+	or edx, 0x10 ; kernel code segment
+	wrmsr
+
+	; syscall rip
+	mov rax, qword syscall_entry
+	mov rdx, qword syscall_entry
+	shr rdx, 32
+	mov ecx, LSTAR
+	wrmsr
+
+	; rflags mask
+	mov ecx, FMASK
+	rdmsr
+	mov eax, ~0x202
+	wrmsr
+
+	; turn on the enable bit (bit 1 of EFER, iirc)
+	call enable_syscall
+	ret
+	
+; still using user stack and memory map
+; can't use user's stack because user may lie about where it is
+; causing us to overwrite memory.
+
+extern sleep
+extern timer_ticks
+
+syscall_entry:
+	mov dr0, rcx
+	mov dr1, rax
+	mov dr2, rdi
+
+	; TODO switch to kernel stack
+
+	; save values for sysret
+	push r11
+	push rcx
+	mov r10, rcx ; fourth arg in r10, since syscall clobbers rcx
+
+	push rax
+	push rdi
+	push rsi
+	mov r8, rdi
+	mov r9, rsi
+
+	mov rdi, qword sleepstr
+	mov rdx, qword timer_ticks
+	mov rsi, [rdx]
+	mov edx, 0
+	mov ecx, eax
+
+	call printk
+	pop rsi
+	pop rdi
+	pop rax
+
+	call sleep
+
+	; syscall number in rax
+	; rcx would be the fourth argument, but over-written by syscall
+	; so we pass it in r10, another scratch register
+
+	; may not need to check for schedule, the syscall itself
+	; should do a yield if it wants to
+
+	; need to check for schedule, could possible just do
+	; an interrupt if it needed scheduling
+	call need_schedule
+	mov dr1, rax
+	;call allstop
+	test eax,eax
+	jz .nosched
+	mov eax, 0x1123f982
+	mov dr0, rax
+	call allstop
+	int 0x81 ; force a schedule call, any int would do actually
+.nosched:
+
+	mov eax, __LINE__
+	mov dr3, rax
+	mov dr2, r11
+	mov dr1, rcx
+	;call allstop
+
+	pop rcx
+	pop r11
+	db 0x48 ; no sysretq in yasm
+	sysret
+
+ign:
+db	'ignoring syscall %u', 0xa, 0
+
+sleepstr:
+	db '%u pid %u syscall %u : %u %u', 0xa, 0
+
-- 
2.40.0