Save and restore es, fs, gs segments in interrupts and context switch.

Remove workaround from sysstat for es register.

kernel/kernelcore.S

@@ -176,12 +176,12 @@ outb %al, $0x92
 	mov	%eax, %cr0		# store the status word
 					# (we are now in protected mode)
 	mov	$2*8, %ax		# selector two is flat 4GB data data
-	mov	%ax, %ds		# set stack and data segments to selector two
+	mov	%ax, %ds		# set data, extra, and stack segments to selector two
+	mov	%ax, %es
 	mov	%ax, %ss
 	mov	$5*8, %ax		# set TSS to selector five
 	ltr	%ax
 	mov	$0, %ax			# unused segments are nulled out
-	mov	%ax, %es
 	mov	%ax, %fs
 	mov	%ax, %gs
 	mov	$INTERRUPT_STACK_TOP, %sp    # set up initial C stack
@@ -404,34 +404,42 @@ intr47: pushl $0 ; pushl $47 ; jmp intr_handler
 intr48: pushl $0 ; pushl $48 ; jmp intr_syscall
 
 intr_handler:
-	pushl	%ds
-	pushl	%ebp		# push regs
+	pushl	%ds		# push segment registers
+	pushl	%es
+	pushl	%fs
+	pushl	%gs
+	pushl	%ebp		# push general regs
 	pushl	%edi
 	pushl	%esi
 	pushl	%edx
 	pushl	%ecx
 	pushl	%ebx
 	pushl	%eax
-	pushl	36(%esp)	# push interrupt code
-	pushl	36(%esp)	# push interrupt number
-	movl	$2*8, %eax	# switch to kernel data seg
+	pushl	48(%esp)	# push interrupt code from above
+	pushl	48(%esp)	# push interrupt number from above
+	movl	$2*8, %eax	# switch to kernel data seg and extra seg
 	movl	%eax, %ds
+	movl	%eax, %es
 	call	interrupt_handler
 	addl	$4, %esp	# remove interrupt number
 	addl	$4, %esp	# remove interrupt code
 	jmp	intr_return
 
 intr_syscall:
-	pushl	%ds
-	pushl	%ebp		# push regs
+	pushl	%ds		# push segment registers
+	pushl	%es
+	pushl	%fs
+	pushl	%gs
+	pushl	%ebp		# push general regs
 	pushl	%edi
 	pushl	%esi
 	pushl	%edx
 	pushl	%ecx
 	pushl	%ebx
 	pushl	%eax		# note these *are* the syscall args
-	movl	$2*8, %eax	# switch to kernel data seg
+	movl	$2*8, %eax	# switch to kernel data seg and extra seg
 	movl	%eax, %ds
+	movl	%eax, %es
 	call	syscall_handler
 	addl	$4, %esp	# remove the old eax
 	jmp	syscall_return	
@@ -446,6 +454,9 @@ syscall_return:
 	popl	%esi
 	popl	%edi
 	popl	%ebp
+	popl	%gs
+	popl	%fs
+	popl	%es
 	popl	%ds
 	addl	$4, %esp	# remove interrupt num
 	addl	$4, %esp	# remove detail code

kernel/process.c

@@ -48,6 +48,9 @@ void process_kstack_reset(struct process *p, unsigned entry_point)
 	s->regs2.ebp = (uint32_t) (p->kstack_ptr + 28);
 	s->old_ebp = (uint32_t) (p->kstack_ptr + 32);
 	s->old_eip = (unsigned) intr_return;
+	s->fs = 0;
+	s->gs = 0;
+	s->es = X86_SEGMENT_USER_DATA;
 	s->ds = X86_SEGMENT_USER_DATA;
 	s->cs = X86_SEGMENT_USER_CODE;
 	s->eip = entry_point;

kernel/x86.h

@@ -57,13 +57,21 @@ struct x86_regs {
 };
 
 struct x86_stack {
+	/* Registers saved by process_switch */
 	struct x86_regs regs2;
+	/* Stack frame of process_switch */
 	int32_t old_ebp;
 	int32_t old_eip;
+	/* Pushed by intr_handler in kernelcore */
 	struct x86_regs regs1;
+	int32_t gs;
+	int32_t fs;
+	int32_t es;
 	int32_t ds;
+	/* Pushed by intrXX in kernelcore. */
 	int32_t intr_num;
 	int32_t intr_code;
+	/* Pushed by X86 CPU Hardware. */
 	int32_t eip;
 	int32_t cs;
 	struct x86_eflags eflags;

user/sysstat.c

@@ -11,21 +11,6 @@ See the file LICENSE for details.
 
 int main(int argc, char *argv[])
 {
-	/*
-	This demonstrates (and fixes) a kernel bug.
-	GCC implements the initialization of the structure on the stack like this:
-            rep stos %eax,%es:(%edi)
-        However, basekernel does not automatically set up (or save) the es segment register,
-	and so the operation crashes.  The es register should be set up correctly in kernelcore,
-	saved and restored when processing interrupts/system calls, and also initialized correctly
-	using process_kstack_init.
-	*/
-
-	/* The workaround here is to explicitly set up the es register prior to using it.*/
-
-	asm("mov %ds, %ax");
-	asm("mov %ax, %es");
-
 	struct system_stats s = {0};
 
 	if (syscall_system_stats(&s)) {