Re: Intel P6 vs P7 system call performance

[Daniel Jacobowitz]

On Thu, Dec 19, 2002 at 05:47:55PM -0800, Linus Torvalds wrote:
> 
> 
> On Thu, 19 Dec 2002, Daniel Jacobowitz wrote:
> > >
> > >    (ptrace also doesn't actually allow you to look at the instruction
> > >    contents in high memory, so gdb won't see the instructions in the
> > >    user-mode fast system call trampoline even when it can single-step
> > >    them, and I don't think I'll bother to fix it up).
> >
> > This worries me.  I'm no x86 guru, but I assume the trampoline's setting of
> > the TF bit will kick in right around the following 'ret'.  So the
> > application will stop and GDB won't be able to read the instruction at
> > PC.  I bet that makes it unhappy.
> 
> It doesn't make gdb all that unhappy, everything seems to work fine
> despite the fact that gdb decides it just can't display the instructions.

Yeah; sometimes it will generate that error in the middle of
single-stepping over something larger, though, and it breaks you out of
whatever you were doing.

> > Shouldn't be that hard to fix this up in ptrace, though.
> 
> Or even in user space, since the high pages are all the same in all
> processes (so gdb doesn't even strictly need ptrace, it can just read it's
> _own_ codespace there). But yeah, we could make ptrace aware of the magic
> pages.

I need to get back to my scheduled ptrace cleanups.  Meanwhile, here's
a patch to do this.  Completely untested, like all good patches; but
it's pretty simple.

===== arch/i386/kernel/ptrace.c 1.17 vs edited =====
--- 1.17/arch/i386/kernel/ptrace.c	Wed Nov 27 18:14:11 2002
+++ edited/arch/i386/kernel/ptrace.c	Thu Dec 19 21:33:37 2002
@@ -21,6 +21,7 @@
 #include <asm/processor.h>
 #include <asm/i387.h>
 #include <asm/debugreg.h>
+#include <asm/fixmap.h>
 
 /*
  * does not yet catch signals sent when the child dies.
@@ -196,6 +197,18 @@
 	case PTRACE_PEEKDATA: {
 		unsigned long tmp;
 		int copied;
+
+		/* Allow ptrace to read from the vsyscall page.  */
+		if (addr >= FIXADDR_START && addr < FIXADDR_TOP &&
+		    (addr & 3) == 0) {
+			int idx = virt_to_fix (addr);
+			if (idx == FIX_VSYSCALL) {
+				tmp = * (unsigned long *) addr;
+				ret = put_user (tmp, (unsigned long *) data);
+				break;
+			}
+		}
+			
 
 		copied = access_process_vm(child, addr, &tmp, sizeof(tmp), 0);
 		ret = -EIO;

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer