syscall fixes

arch/x86_64/include/x86_64/defs.h

@@ -117,6 +117,7 @@
 #define CPUID_7D_AMX_INT8           (1 << 25)
 
 #define RFLAGS_CF   (1u << 0)
+#define RFLAGS_BRKI (1u << 1)
 #define RFLAGS_PF   (1u << 2)
 #define RFLAGS_AF   (1u << 4)
 #define RFLAGS_ZF   (1u << 6)

arch/x86_64/percpu.c

@@ -66,10 +66,11 @@ void x86_64_cpu_setup(struct percpu* cpu) {
     asm_wrmsr(MSR_EFER, asm_rdmsr(MSR_EFER) | MSR_EFER_SCE);
     union ia32_star star = {0};
     star.kernel_cs_ss = offsetof(struct gdt, kernel_code64);
-    star.user_cs_ss = offsetof(struct gdt, user_code32);
+    star.user_cs_ss = offsetof(struct gdt, user_code32) | CPL_USER;
     asm_wrmsr(MSR_STAR, star.value);
     asm_wrmsr(MSR_LSTAR, (uint64_t)arch_syscall_stub);
-    asm_wrmsr(MSR_SFMASK, RFLAGS_AC | RFLAGS_DF | RFLAGS_IF);
+    // Mask all RFLAGS except for BRKI which is reserved.
+    asm_wrmsr(MSR_SFMASK, ~RFLAGS_BRKI);
 
     // TODO: FRED
 

arch/x86_64/syscall.c

@@ -58,8 +58,10 @@ void arch_syscall_stub() {
         "    pop rbx\n"
         "    pop rax\n"
         "    add rsp, 0x10\n" // Skip .error and .isr fields
-        "2:\n"
-        "    mov rsp, gs:%c[user_stack]\n"
+        "    pop rcx\n"       // rip
+        "    add rsp, 0x08\n" // Skip cs
+        "    pop r11\n"       // rflags
+        "    pop rsp\n"       // rsp
         "    swapgs\n"
         "    sysretq\n"
         :