Re: Intel P6 vs P7 system call performance

From: Daniel Jacobowitz (dan@debian.org)
Date: Thu Dec 19 2002 - 21:37:36 EST


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
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