sched: implement user-configurable fs and gs segment base addresses

arch/x86_64/asm-offset.c

@@ -28,4 +28,6 @@
 static void __used common(void)
 {
 	OFFSET(THREAD_sp, struct thread, tr_sp);
+	OFFSET(THREAD_fsbase, struct thread, tr_ml.tr_fsbase);
+	OFFSET(THREAD_gsbase, struct thread, tr_ml.tr_gsbase);
 }

arch/x86_64/include/arch/msr.h

@@ -3,6 +3,7 @@
 
 #include <stdint.h>
 
+#define MSR_FS_BASE        0xC0000100
 #define MSR_GS_BASE        0xC0000101
 #define MSR_KERNEL_GS_BASE 0xC0000102
 

arch/x86_64/include/kernel/machine/thread.h

@@ -3,6 +3,10 @@
 
 #include <kernel/sched.h>
 
+struct ml_thread {
+	virt_addr_t tr_gsbase, tr_fsbase;
+};
+
 struct ml_cpu_context;
 
 /* switch from one thread to another. the stack of the `to` thread must have
@@ -28,4 +32,15 @@ extern kern_status_t ml_thread_prepare_user_context(
 	const uintptr_t *args,
 	size_t nr_args);
 
+extern kern_status_t ml_thread_config_get(
+	struct thread *thread,
+	kern_config_key_t key,
+	void *out,
+	size_t max);
+extern kern_status_t ml_thread_config_set(
+	struct thread *thread,
+	kern_config_key_t key,
+	const void *ptr,
+	size_t len);
+
 #endif

arch/x86_64/thread.c

@@ -1,5 +1,7 @@
+#include <arch/msr.h>
 #include <kernel/machine/cpu.h>
 #include <kernel/machine/thread.h>
+#include <kernel/thread.h>
 
 #define MAX_REG_ARGS 6
 #define REG_ARG_0    rdi
@@ -77,3 +79,52 @@ extern kern_status_t ml_thread_prepare_user_context(
 
 	return KERN_OK;
 }
+
+kern_status_t ml_thread_config_get(
+	struct thread *thread,
+	kern_config_key_t key,
+	void *out,
+	size_t max)
+{
+	return KERN_OK;
+}
+
+kern_status_t ml_thread_config_set(
+	struct thread *thread,
+	kern_config_key_t key,
+	const void *ptr,
+	size_t len)
+{
+	switch (key) {
+	case THREAD_CFG_FSBASE:
+		if (len != sizeof(thread->tr_ml.tr_fsbase)) {
+			return KERN_INVALID_ARGUMENT;
+		}
+
+		thread->tr_ml.tr_fsbase = *(virt_addr_t *)ptr;
+		if (thread == current_thread()) {
+			wrmsr(MSR_FS_BASE, thread->tr_ml.tr_fsbase);
+		}
+
+		break;
+	case THREAD_CFG_GSBASE:
+		if (len != sizeof(thread->tr_ml.tr_gsbase)) {
+			return KERN_INVALID_ARGUMENT;
+		}
+
+		thread->tr_ml.tr_gsbase = *(virt_addr_t *)ptr;
+		if (thread == current_thread()) {
+			/* we're in the kernel right now, so the user and kernel
+			 * gs-base registers are swapped. when we return to
+			 * usermode, this value will be swapped back into
+			 * the user gs-base register */
+			wrmsr(MSR_KERNEL_GS_BASE, thread->tr_ml.tr_gsbase);
+		}
+
+		break;
+	default:
+		return KERN_INVALID_ARGUMENT;
+	}
+
+	return KERN_OK;
+}

arch/x86_64/thread_switch.S

@@ -13,6 +13,22 @@ ml_thread_switch:
 	push %rax
 	push %rcx
 	push %rdx
+
+	// set fs-base
+	mov $0xC0000100, %rcx
+	movq THREAD_fsbase(%rsi), %rax
+	movq THREAD_fsbase(%rsi), %rdx
+	shr $32, %rdx
+	wrmsr
+
+	// set (kernel) gs-base (it will be swapped back into user-gs-base at
+	// the end of this function)
+	mov $0xC0000102, %rcx
+	movq THREAD_gsbase(%rsi), %rax
+	movq THREAD_gsbase(%rsi), %rdx
+	shr $32, %rdx
+	wrmsr
+
 	push %rbx
 	pushq $0
 	push %rbp