[PATCH] uml: Use PTRACE_SCEMU (the so-called SYSEMU) to reduce syscall cost.

Turns off syscall emulation patch for ptrace (SYSEMU) on.  SYSEMU is a
performance-patch introduced by Laurent Vivier.  It changes behaviour of
ptrace() and helps reducing host context switch rate.  To make it working, you
need a kernel patch for your host, too.  See
http://perso.wanadoo.fr/laurent.vivier/UML/ for further information.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/kernel/process.c

@@ -19,6 +19,7 @@
 #include <asm/sigcontext.h>
 #include <asm/unistd.h>
 #include <asm/page.h>
+#include <asm/user.h>
 #include "user_util.h"
 #include "kern_util.h"
 #include "user.h"
@@ -227,6 +228,40 @@ void __init check_ptrace(void)
 	}
 	stop_ptraced_child(pid, stack, 0);
 	printk("OK\n");
+
+#ifdef PTRACE_SYSEMU
+	printk("Checking syscall emulation patch for ptrace...");
+	use_sysemu = 0;
+	pid = start_ptraced_child(&stack);
+	if(ptrace(PTRACE_SYSEMU, pid, 0, 0) >= 0) {
+		struct user_regs_struct regs;
+
+		if (waitpid(pid, &status, WUNTRACED) < 0)
+			panic("check_ptrace : wait failed, errno = %d", errno);
+		if(!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGTRAP))
+			panic("check_ptrace : expected SIGTRAP, "
+			      "got status = %d", status);
+
+		if (ptrace(PTRACE_GETREGS, pid, 0, &regs) < 0)
+			panic("check_ptrace : failed to read child "
+			      "registers, errno = %d", errno);
+		regs.orig_eax = pid;
+		if (ptrace(PTRACE_SETREGS, pid, 0, &regs) < 0)
+			panic("check_ptrace : failed to modify child "
+			      "registers, errno = %d", errno);
+
+		stop_ptraced_child(pid, stack, 0);
+
+		printk("OK\n");
+		use_sysemu = 1;
+	}
+	else
+	{
+		printk("missing\n");
+		stop_ptraced_child(pid, stack, 1);
+	}
+
+# endif /* PTRACE_SYSEMU */
 }
 
 int run_kernel_thread(int (*fn)(void *), void *arg, void **jmp_ptr)

arch/um/kernel/skas/include/ptrace-skas.h

@@ -10,6 +10,13 @@
 
 #ifdef UML_CONFIG_MODE_SKAS
 
+/* syscall emulation path in ptrace */
+
+#ifndef PTRACE_SYSEMU
+#define PTRACE_SYSEMU 31
+#endif
+extern int use_sysemu;
+
 #include "skas_ptregs.h"
 
 #define HOST_FRAME_SIZE 17

arch/um/kernel/skas/process.c

@@ -28,6 +28,10 @@
 #include "chan_user.h"
 #include "signal_user.h"
 
+#ifdef PTRACE_SYSEMU
+int use_sysemu = 0;
+#endif
+
 int is_skas_winch(int pid, int fd, void *data)
 {
 	if(pid != getpid())
@@ -68,6 +72,10 @@ static void handle_trap(int pid, union uml_pt_regs *regs)
 		return;
 	}
 
+#ifdef PTRACE_SYSEMU
+	if (!use_sysemu)
+	{
+#endif
 	err = ptrace(PTRACE_POKEUSER, pid, PT_SYSCALL_NR_OFFSET, __NR_getpid);
 	if(err < 0)
 	        panic("handle_trap - nullifying syscall failed errno = %d\n", 
@@ -82,6 +90,9 @@ static void handle_trap(int pid, union uml_pt_regs *regs)
 	if((err < 0) || !WIFSTOPPED(status) || (WSTOPSIG(status) != SIGTRAP))
 		panic("handle_trap - failed to wait at end of syscall, "
 		      "errno = %d, status = %d\n", errno, status);
+#ifdef PTRACE_SYSEMU
+	}
+#endif
 
 	handle_syscall(regs);
 }
@@ -139,6 +150,11 @@ void userspace(union uml_pt_regs *regs)
 
 	restore_registers(regs);
 		
+#ifdef PTRACE_SYSEMU
+	if (use_sysemu)
+		err = ptrace(PTRACE_SYSEMU, pid, 0, 0);
+	else
+#endif
 	err = ptrace(PTRACE_SYSCALL, pid, 0, 0);
 	if(err)
 		panic("userspace - PTRACE_SYSCALL failed, errno = %d\n", 
@@ -177,6 +193,12 @@ void userspace(union uml_pt_regs *regs)
 
 		restore_registers(regs);
 
+#ifdef PTRACE_SYSEMU
+		if (use_sysemu)
+			op = singlestepping_skas() ? PTRACE_SINGLESTEP :
+				PTRACE_SYSEMU;
+		else
+#endif
 		op = singlestepping_skas() ? PTRACE_SINGLESTEP : 
 			PTRACE_SYSCALL;
 		err = ptrace(op, pid, 0, 0);