diff --git a/arch/um/include/kern_util.h b/arch/um/include/kern_util.h
index 07176d92e1c92789daf5c75df5c901d7605bd5c5..42557130a4081868e7d4c3cfc8cf9a5e8514d7db 100644
--- a/arch/um/include/kern_util.h
+++ b/arch/um/include/kern_util.h
@@ -116,7 +116,11 @@ extern void *get_current(void);
 extern struct task_struct *get_task(int pid, int require);
 extern void machine_halt(void);
 extern int is_syscall(unsigned long addr);
-extern void arch_switch(void);
+
+extern void arch_switch_to_tt(struct task_struct *from, struct task_struct *to);
+
+extern void arch_switch_to_skas(struct task_struct *from, struct task_struct *to);
+
 extern void free_irq(unsigned int, void *);
 extern int cpu(void);
 
diff --git a/arch/um/include/sysdep-i386/ptrace.h b/arch/um/include/sysdep-i386/ptrace.h
index c8ee9559f3ab2d3994fc83e424627b9c53993607..6670cc992ecb55de9c98e9ca19131c6c1a556592 100644
--- a/arch/um/include/sysdep-i386/ptrace.h
+++ b/arch/um/include/sysdep-i386/ptrace.h
@@ -14,7 +14,12 @@
 #define MAX_REG_NR (UM_FRAME_SIZE / sizeof(unsigned long))
 #define MAX_REG_OFFSET (UM_FRAME_SIZE)
 
+#ifdef UML_CONFIG_PT_PROXY
 extern void update_debugregs(int seq);
+#else
+static inline void update_debugregs(int seq) {}
+#endif
+
 
 /* syscall emulation path in ptrace */
 
diff --git a/arch/um/kernel/skas/process_kern.c b/arch/um/kernel/skas/process_kern.c
index 3f70a2e12f067504f48243a204190ff2eff44027..14360ac17f028d33d72082229ade0fa6389ce7f2 100644
--- a/arch/um/kernel/skas/process_kern.c
+++ b/arch/um/kernel/skas/process_kern.c
@@ -35,6 +35,8 @@ void switch_to_skas(void *prev, void *next)
 	switch_threads(&from->thread.mode.skas.switch_buf,
 		       to->thread.mode.skas.switch_buf);
 
+	arch_switch_to_skas(current->thread.prev_sched, current);
+
 	if(current->pid == 0)
 		switch_timers(1);
 }
diff --git a/arch/um/kernel/tt/process_kern.c b/arch/um/kernel/tt/process_kern.c
index 295c1ac817b34cc6f8d04aeab783539b5004060a..a9c1443fc5481fedf27813f203721690c8996590 100644
--- a/arch/um/kernel/tt/process_kern.c
+++ b/arch/um/kernel/tt/process_kern.c
@@ -51,6 +51,13 @@ void switch_to_tt(void *prev, void *next)
 
 	c = 0;
 
+	/* Notice that here we "up" the semaphore on which "to" is waiting, and
+	 * below (the read) we wait on this semaphore (which is implemented by
+	 * switch_pipe) and go sleeping. Thus, after that, we have resumed in
+	 * "to", and can't use any more the value of "from" (which is outdated),
+	 * nor the value in "to" (since it was the task which stole us the CPU,
+	 * which we don't care about). */
+
 	err = os_write_file(to->thread.mode.tt.switch_pipe[1], &c, sizeof(c));
 	if(err != sizeof(c))
 		panic("write of switch_pipe failed, err = %d", -err);
@@ -77,7 +84,7 @@ void switch_to_tt(void *prev, void *next)
 	change_sig(SIGALRM, alrm);
 	change_sig(SIGPROF, prof);
 
-	arch_switch();
+	arch_switch_to_tt(prev_sched, current);
 
 	flush_tlb_all();
 	local_irq_restore(flags);
@@ -141,7 +148,6 @@ static void new_thread_handler(int sig)
 	set_cmdline("(kernel thread)");
 
 	change_sig(SIGUSR1, 1);
-	change_sig(SIGVTALRM, 1);
 	change_sig(SIGPROF, 1);
 	local_irq_enable();
 	if(!run_kernel_thread(fn, arg, &current->thread.exec_buf))
diff --git a/arch/um/sys-i386/ptrace.c b/arch/um/sys-i386/ptrace.c
index ff94eded93ebf537b555030f3aaae3566e0bc67c..927fcd955651367aff76562abf6f3a47d1d52886 100644
--- a/arch/um/sys-i386/ptrace.c
+++ b/arch/um/sys-i386/ptrace.c
@@ -15,9 +15,13 @@
 #include "sysdep/sigcontext.h"
 #include "sysdep/sc.h"
 
-void arch_switch(void)
+void arch_switch_to_tt(struct task_struct *from, struct task_struct *to)
+{
+	update_debugregs(to->thread.arch.debugregs_seq);
+}
+
+void arch_switch_to_skas(struct task_struct *from, struct task_struct *to)
 {
-	update_debugregs(current->thread.arch.debugregs_seq);
 }
 
 int is_syscall(unsigned long addr)
diff --git a/arch/um/sys-i386/ptrace_user.c b/arch/um/sys-i386/ptrace_user.c
index 7c376c95de506e2d0bd71070045b64af1d52f7e0..9f3bd8ed78f5b2d7cc47380227051327aad98e3b 100644
--- a/arch/um/sys-i386/ptrace_user.c
+++ b/arch/um/sys-i386/ptrace_user.c
@@ -14,6 +14,7 @@
 #include "sysdep/thread.h"
 #include "user.h"
 #include "os.h"
+#include "uml-config.h"
 
 int ptrace_getregs(long pid, unsigned long *regs_out)
 {
@@ -43,6 +44,7 @@ int ptrace_setfpregs(long pid, unsigned long *regs)
 	return 0;
 }
 
+/* All the below stuff is of interest for TT mode only */
 static void write_debugregs(int pid, unsigned long *regs)
 {
 	struct user *dummy;
@@ -75,7 +77,6 @@ static void read_debugregs(int pid, unsigned long *regs)
 
 /* Accessed only by the tracing thread */
 static unsigned long kernel_debugregs[8] = { [ 0 ... 7 ] = 0 };
-static int debugregs_seq = 0;
 
 void arch_enter_kernel(void *task, int pid)
 {
@@ -89,6 +90,11 @@ void arch_leave_kernel(void *task, int pid)
 	write_debugregs(pid, TASK_DEBUGREGS(task));
 }
 
+#ifdef UML_CONFIG_PT_PROXY
+/* Accessed only by the tracing thread */
+static int debugregs_seq;
+
+/* Only called by the ptrace proxy */
 void ptrace_pokeuser(unsigned long addr, unsigned long data)
 {
 	if((addr < offsetof(struct user, u_debugreg[0])) ||
@@ -109,6 +115,7 @@ static void update_debugregs_cb(void *arg)
 	write_debugregs(pid, kernel_debugregs);
 }
 
+/* Optimized out in its header when not defined */
 void update_debugregs(int seq)
 {
 	int me;
@@ -118,6 +125,7 @@ void update_debugregs(int seq)
 	me = os_getpid();
 	initial_thread_cb(update_debugregs_cb, &me);
 }
+#endif
 
 /*
  * Overrides for Emacs so that we follow Linus's tabbing style.