Seems like the problem with syscall/IF is fixed

etc/init.S

@@ -1,10 +1,16 @@
 .section .text
 .global _start
 _start:
-    movq $100, %rcx
+0:
+    movq $1, %rdi
+    movq $2, %rsi
+    movq $3, %rdx
+    movq $4, %r10
+    movq $5, %r8
+    movq $6, %r9
+    syscall
+    movq $10000000, %rcx
 1:
     dec %rcx
     jnz 1b
-    // XXX: Syscall fucks up kstack?
-    //syscall
-    jmp _start
+    jmp 0b

include/sys/amd64/hw/int_macros.inc

@@ -41,11 +41,12 @@
     popq %rax
     mov %rax, %ds
     popq %rax
-    //mov %rax, %es
+    mov %rax, %es
     popq %rax
-    //mov %rax, %fs
     popq %rax
-    //mov %rax, %gs
+    xor %rax, %rax
+    mov %rax, %fs
+    mov %rax, %gs
 
     // cr3
     popq %rax

sys/amd64/sys/syscall.c

@@ -2,4 +2,8 @@
 #include "sys/debug.h"
 
 void amd64_syscall(uint64_t a0, uint64_t a1, uint64_t a2, uint64_t a3, uint64_t a4, uint64_t a5) {
+    switch (a0) {
+    default:
+        return;
+    }
 }

sys/amd64/sys/syscall_s.S

@@ -28,7 +28,7 @@ amd64_setup_syscall:
     movl $AMD64_MSR_LSTAR, %ecx
     wrmsr
 
-    movl $0x0, %eax
+    movl $0x0200, %eax
     movl $0x0, %edx
     movl $AMD64_MSR_SFMASK, %ecx
     wrmsr
@@ -42,7 +42,6 @@ amd64_setup_syscall:
     retq
 
 amd64_syscall_entry:
-    cli
     // R11 = RFLAGS
     // RCX = userRIP
     // RSP = userRSP
@@ -54,8 +53,8 @@ amd64_syscall_entry:
     // 1. Load kernRSP from amd64_thread_current->kstack_ptr,
     //    userRSP is stored in %rax before being pushed
     movq %rsp, %rax
-    movq sched_current(%rip), %rsi
-    movq (%rsi), %rsp
+    movq sched_current(%rip), %rsp
+    movq (%rsp), %rsp
 
     // syscall handlers may mess up this in SysV amd64 ABI, store
     // these on stack
@@ -63,6 +62,9 @@ amd64_syscall_entry:
     pushq %r11
     pushq %rcx
 
+    // Move arguments
+    movq %r10, %rcx
+
     call amd64_syscall
 
     popq %rcx

sys/amd64/sys/thread.c

@@ -31,9 +31,11 @@ int thread_init(thread_t *t,
             panic("Failed to allocate thread kstack\n");
         }
     }
+
     t->kstack_base = kstack_base;
     t->kstack_size = kstack_size;
     t->kstack_ptr = t->kstack_base + t->kstack_size - sizeof(amd64_thread_context_t);
+    kdebug("KSTACK %p\n", t->kstack_ptr);
 
     // If we're not kernel, additionally set the ustack
     if (!(flags & THREAD_KERNEL)) {

sys/sched.c

@@ -44,6 +44,7 @@ int sched(void) {
     if (to == from) {
         return -1;
     }
+    sched_current = to;
 
     return 0;
 }