[kernel] Update syscall MSRs for all CPUs

Since SYSCALL/SYSRET rely on MSRs to control their function, split out
syscall_enable() into syscall_initialize() and syscall_enable(), the
latter being called on all CPUs. This affects not just syscalls but also
the kernel_to_user_trampoline.

Additionally, do away with the max syscalls, and just make a single page
of syscall pointers and name pointers. Max syscalls was fragile and
needed to be kept in sync in multiple places.

src/kernel/cpu.cpp

@@ -11,6 +11,7 @@
 #include "log.h"
 #include "msr.h"
 #include "objects/vm_area.h"
+#include "syscall.h"
 #include "tss.h"
 
 cpu_data g_bsp_cpu_data;
@@ -98,6 +99,8 @@ init_cpu(bool bsp)
 
 		tss->ist_stack(ist) = stack_top;
 	}
+	// Set up the syscall MSRs
+	syscall_enable();
 
 	// Set up the page attributes table
 	uint64_t pat = rdmsr(msr::ia32_pat);

src/kernel/main.cpp

@@ -149,6 +149,7 @@ kernel_main(args::header *header)
 		}
 	}
 
+	syscall_initialize();
 
 	device_manager &devices = device_manager::get();
 	devices.parse_acpi(header->acpi_table);
@@ -184,7 +185,6 @@ kernel_main(args::header *header)
 	}
 	*/
 
-	syscall_enable();
 	scheduler *sched = new scheduler(devices.get_lapic());
 
 	// Skip program 0, which is the kernel itself

src/kernel/syscall.cpp

@@ -10,11 +10,11 @@
 
 extern "C" {
 	void syscall_invalid(uint64_t call);
-	void syscall_handler_prelude();
 }
 
-uintptr_t syscall_registry[static_cast<unsigned>(syscall::MAX)];
+uintptr_t syscall_registry[256] __attribute__((section(".syscall_registry")));
-const char * syscall_names[static_cast<unsigned>(syscall::MAX)];
+const char * syscall_names[256] __attribute__((section(".syscall_registry")));
+static constexpr size_t num_syscalls = sizeof(syscall_registry) / sizeof(syscall_registry[0]);
 
 void
 syscall_invalid(uint64_t call)
@@ -23,13 +23,10 @@ syscall_invalid(uint64_t call)
 	cons->set_color(9);
 	cons->printf("\nReceived unknown syscall: %02x\n", call);
 
-	const unsigned num_calls =
-		static_cast<unsigned>(syscall::MAX);
-
 	cons->printf("  Known syscalls:\n");
 		cons->printf("          invalid %016lx\n", syscall_invalid);
 
-	for (unsigned i = 0; i < num_calls; ++i) {
+	for (unsigned i = 0; i < num_syscalls; ++i) {
 		const char *name = syscall_names[i];
 		uintptr_t handler = syscall_registry[i];
 		if (name)
@@ -41,33 +38,14 @@ syscall_invalid(uint64_t call)
 }
 
 void
-syscall_enable()
+syscall_initialize()
 {
-	// IA32_STAR - high 32 bits contain k+u CS
-	// Kernel CS: GDT[1] ring 0 bits[47:32]
-	//   User CS: GDT[3] ring 3 bits[63:48]
-	uint64_t star =
-		(((1ull << 3) | 0) << 32) |
-		(((3ull << 3) | 3) << 48);
-	wrmsr(msr::ia32_star, star);
-
-	// IA32_LSTAR - RIP for syscall
-	wrmsr(msr::ia32_lstar,
-		reinterpret_cast<uintptr_t>(&syscall_handler_prelude));
-
-	// IA32_FMASK - FLAGS mask inside syscall
-	wrmsr(msr::ia32_fmask, 0x200);
-
-	static constexpr unsigned num_calls =
-		static_cast<unsigned>(syscall::MAX);
-
 	kutil::memset(&syscall_registry, 0, sizeof(syscall_registry));
 	kutil::memset(&syscall_names, 0, sizeof(syscall_names));
 
 #define SYSCALL(id, name, result, ...) \
 	syscall_registry[id] = reinterpret_cast<uintptr_t>(syscalls::name); \
 	syscall_names[id] = #name; \
-	static_assert( id <= num_calls, "Syscall " #name " has id > syscall::MAX" ); \
 	log::debug(logs::syscall, "Enabling syscall 0x%02x as " #name , id);
 #include "j6/tables/syscalls.inc"
 #undef SYSCALL

src/kernel/syscall.h

@@ -10,13 +10,10 @@ enum class syscall : uint64_t
 #define SYSCALL(id, name, ...) name = id,
 #include "j6/tables/syscalls.inc"
 #undef SYSCALL
-
-	// Maximum syscall id. If you change this, also change
-	// MAX_SYSCALLS in syscall.s
-	MAX = 0x40
 };
 
-void syscall_enable();
+void syscall_initialize();
+extern "C" void syscall_enable();
 
 namespace syscalls
 {

src/kernel/syscall.s

@@ -1,17 +1,32 @@
 %include "tasking.inc"
 
-; Make sure to keep MAX_SYSCALLS in sync with
+; SYSCALL/SYSRET control MSRs
-; syscall::MAX in syscall.h
+MSR_STAR   equ 0xc0000081
-MAX_SYSCALLS equ 0x40
+MSR_LSTAR  equ 0xc0000082
+MSR_FMASK  equ 0xc0000084
+
+; IA32_STAR - high 32 bits contain k+u CS
+; Kernel CS: GDT[1] ring 0 bits[47:32]
+;   User CS: GDT[3] ring 3 bits[63:48]
+STAR_HIGH  equ \
+	(((1 << 3) | 0)) | \
+	(((3 << 3) | 3) << 16)
+
+; IA32_FMASK - Mask off interrupts in syscalls
+FMASK_VAL  equ 0x200
 
 extern __counter_syscall_enter
 extern __counter_syscall_sysret
-
 extern syscall_registry
 extern syscall_invalid
 
-global syscall_handler_prelude
+
+global syscall_handler_prelude:function (syscall_handler_prelude.end - syscall_handler_prelude)
 syscall_handler_prelude:
+	push rbp     ; Never executed, fake function prelude
+	mov rbp, rsp ; to calm down gdb
+
+.real:
 	swapgs
 	mov [gs:CPU_DATA.rsp3], rsp
 	mov rsp, [gs:CPU_DATA.rsp0]
@@ -36,14 +51,7 @@ syscall_handler_prelude:
 
 	inc qword [rel __counter_syscall_enter]
 
-	cmp rax, MAX_SYSCALLS
+	and rax, 0xff ; Only 256 possible syscall values
-	jle .ok_syscall
-
-.bad_syscall:
-	mov rdi, rax
-	call syscall_invalid
-
-.ok_syscall:
 	lea r11, [rel syscall_registry]
 	mov r11, [r11 + rax * 8]
 	cmp r11, 0
@@ -52,8 +60,14 @@ syscall_handler_prelude:
 	call r11
 
 	inc qword [rel __counter_syscall_sysret]
+	jmp kernel_to_user_trampoline
 
-global kernel_to_user_trampoline
+.bad_syscall:
+	mov rdi, rax
+	call syscall_invalid
+.end:
+
+global kernel_to_user_trampoline:function (kernel_to_user_trampoline.end - kernel_to_user_trampoline)
 kernel_to_user_trampoline:
 	pop r15
 	pop r14
@@ -70,3 +84,28 @@ kernel_to_user_trampoline:
 
 	swapgs
 	o64 sysret
+.end:
+
+global syscall_enable:function (syscall_enable.end - syscall_enable)
+syscall_enable:
+	push rbp
+	mov rbp, rsp
+
+	mov rcx, MSR_STAR
+	mov rax, 0
+	mov rdx, STAR_HIGH
+	wrmsr
+
+	mov rcx, MSR_LSTAR
+	mov rax, syscall_handler_prelude.real
+	mov rdx, rax
+	shr rdx, 32
+	wrmsr
+
+	mov rcx, MSR_FMASK
+	mov rax, FMASK_VAL
+	wrmsr
+
+	pop rbp
+	ret
+.end: