From ad9ba944d10d6c068a1af6aecd1f0afea875ae1c Mon Sep 17 00:00:00 2001
From: Tomas Hruby <tom@minix3.org>
Date: Tue, 9 Feb 2010 15:13:52 +0000
Subject: [PATCH] Early address space switch

- switch_address_space() implements a switch of the user address space
  for the destination process

- this makes memory of this process easily accessible, e.g. a pointer
  valid in the userspace can be used with a little complexity to
  access the process's memory

- the switch does not happed only just before we return to userspace,
  however, it happens right after we know which process we are going
  to schedule. This happens before we start processing the misc flags
  of this process so its memory is available

- if the process becomes not runnable while processing the mics flags
  we pick a new process and we switch the address space again which
  introduces possibly a little bit more overhead, however, it is
  hopefully hidden by reducing the overheads when we actually access
  the memory
---
 kernel/arch/i386/klib386.S | 36 ++++++++++++++++++++++++++++++++++++
 kernel/arch/i386/mpx386.S  | 17 -----------------
 kernel/arch/i386/system.c  |  3 ---
 kernel/proc.c              |  2 ++
 kernel/proto.h             |  1 +
 5 files changed, 39 insertions(+), 20 deletions(-)

diff --git a/kernel/arch/i386/klib386.S b/kernel/arch/i386/klib386.S
index 6e136cc11..852fa6c8b 100644
--- a/kernel/arch/i386/klib386.S
+++ b/kernel/arch/i386/klib386.S
@@ -692,3 +692,39 @@ idt_reload:
 reload_ds:
 	RELOAD_SEG_REG(%ds)
 	ret
+
+/*===========================================================================*/
+/*			      switch_address_space			     */
+/*===========================================================================*/
+/* PUBLIC void switch_address_space(struct proc *p)
+ *
+ * sets the %cr3 register to the supplied value if it is not already set to the
+ * same value in which case it would only result in an extra TLB flush which is
+ * not desirable
+ */
+ .balign 16
+.globl switch_address_space
+switch_address_space:
+
+	/* read the process pointer */
+	mov	4(%esp), %edx
+	/* enable process' segment descriptors  */
+	lldt	P_LDT_SEL(%edx)
+	/* get the new cr3 value */
+	movl	P_CR3(%edx), %eax
+	/* test if the new cr3 != NULL */
+	cmpl	$0, %eax
+	je	0f
+
+	/*
+	 * test if the cr3 is loaded with the current value to avoid unnecessary
+	 * TLB flushes
+	 */
+	mov	%cr3, %ecx
+	cmp	%ecx, %eax
+	je	0f
+	mov	%eax, %cr3
+	mov	%edx, ptproc
+	movl	$0, dirtypde
+0:
+	ret
diff --git a/kernel/arch/i386/mpx386.S b/kernel/arch/i386/mpx386.S
index d67ba0acc..321bc59a5 100644
--- a/kernel/arch/i386/mpx386.S
+++ b/kernel/arch/i386/mpx386.S
@@ -470,22 +470,6 @@ restart:
 	/* %eax is set by schedcheck() to the process to run */
 	mov	%eax, %ebp	/* will assume P_STACKBASE == 0 */
 
-	cmpl	$0, P_CR3(%ebp)
-	jz	0f
-
-	/*
-	 * test if the cr3 is loaded with the current value to avoid unnecessary
-	 * TLB flushes
-	 */
-	mov	P_CR3(%ebp), %eax
-	mov	%cr3, %ecx
-	cmp	%ecx, %eax
-	jz	0f
-	mov	%eax, %cr3
-	mov	%ebp, ptproc
-	movl	$0, dirtypde
-0:
-
 	/* reconstruct the stack for iret */
 	movl	SSREG(%ebp), %eax
 	push	%eax
@@ -500,7 +484,6 @@ restart:
 
 	RESTORE_GP_REGS(%ebp)
 
-	lldt	P_LDT_SEL(%ebp)	/* enable process' segment descriptors  */
 	RESTORE_SEGS(%ebp)
 
 	movl	%ss:BPREG(%ebp), %ebp
diff --git a/kernel/arch/i386/system.c b/kernel/arch/i386/system.c
index 703056cc3..d857eeece 100644
--- a/kernel/arch/i386/system.c
+++ b/kernel/arch/i386/system.c
@@ -477,9 +477,6 @@ PUBLIC void arch_do_syscall(struct proc *proc)
   m_ptr = (message *) proc->p_reg.bx;
   bit_map = proc->p_reg.dx;
 
-  /* do_ipc() expects the given process's memory to be accessible. */
-  vm_set_cr3(proc);
-
   /* Make the system call, for real this time. */
   proc->p_reg.retreg = do_ipc(call_nr, src_dst_e, m_ptr, bit_map);
 }
diff --git a/kernel/proc.c b/kernel/proc.c
index 99a2b202d..04062a105 100644
--- a/kernel/proc.c
+++ b/kernel/proc.c
@@ -198,6 +198,8 @@ not_runnable_pick_new:
 		idle();
 	}
 
+	switch_address_space(proc_ptr);
+
 check_misc_flags:
 
 	vmassert(proc_ptr);
diff --git a/kernel/proto.h b/kernel/proto.h
index 69c4d9f07..5226727a4 100644
--- a/kernel/proto.h
+++ b/kernel/proto.h
@@ -171,4 +171,5 @@ _PROTOTYPE( int arch_phys_map, (int index, phys_bytes *addr,
 _PROTOTYPE( int arch_phys_map_reply, (int index, vir_bytes addr));
 _PROTOTYPE( int arch_enable_paging, (void));
 
+_PROTOTYPE(void switch_address_space, (struct proc * p));
 #endif /* PROTO_H */