[Crash-utility] [PATCH] Make the __per_cpu_offset symbol available

[Petr Tesarik]

On Friday 07 of January 2011 16:20:15 Dave Anderson wrote:
> ----- Original Message -----
>
> > Hi,
> >
> > the code in xen_hyper_init() tries to determine the per-cpu shift from
> > the __per_cpu_shift symbol if available. This doesn't work because the
> > symbol's value is outside the kernel text address range. This patch makes
> > a special case for this symbol on Xen targets, so it can be used later.
>
> Hi Petr,

Hi Dave,

> The patch looks OK -- but it seems to be a bit of overkill adding a bunch
> of new functions?  Couldn't each of the original functions simply have
> a check like this right at the top?:
>
>      if (XEN_HYPER_MODE() && STREQ(symbol, "__per_cpu_shift"))
>          return TRUE;

Er, yes. When I started the patch, I thought I would need some more special 
handling, but it then boiled down to just this one symbol, so you're right, 
it works.

> > Without this patch, crash initialization fails on SLES11 Xen hypervisor
> > dumps, because on incorrect per-cpu offset is assumed. The failure can't
> > be fixed simply by adding a version check, because SLES11 Xen is patched
> > to increase the per-cpu shift in order to support more CPUs.
> >
> > It is also important not to try to relocate ABSOLUTE symbol (such as this
> > one), so add this condition in store_symbols().
>
> The reloc stuff is only applicable to live x86/x86_64 vmlinux sessions.  To
> absolutely ensure backwards-compatibility, I'd rather just put this at the
> top of relocate():
>
>      if (XEN_HYPER_MODE())
>              return symval;
>
> Would that work for you?

Yes, sure. As long as we don't have live sessions on relocatable Xen 
hypervisors. Which isn't coming... Here's the shorter patch:

Signed-off-by: Petr Tesarik <ptesarik at suse.cz>
---
 ia64.c    |    3 +++
 symbols.c |    3 +++
 x86.c     |    3 +++
 x86_64.c  |    3 +++
 4 files changed, 12 insertions(+)

--- a/ia64.c
+++ b/ia64.c
@@ -711,6 +711,9 @@ ia64_verify_symbol(const char *name, ulo
 	if (!name || !strlen(name))
 		return FALSE;
 
+	if (XEN_HYPER_MODE() && STREQ(name, "__per_cpu_shift"))
+		return TRUE;
+
         if (CRASHDEBUG(8))
                 fprintf(fp, "%016lx %s\n", value, name);
 
--- a/symbols.c
+++ b/symbols.c
@@ -678,6 +678,9 @@ store_sysmap_symbols(void)
 static ulong
 relocate(ulong symval, char *symname, int first_symbol)
 {
+	if (XEN_HYPER_MODE())
+		return symval;
+
 	switch (kt->flags & (RELOC_SET|RELOC_FORCE))
 	{
 	case RELOC_SET: 
--- a/x86.c
+++ b/x86.c
@@ -3785,6 +3785,9 @@ x86_is_task_addr(ulong task)
 static int
 x86_verify_symbol(const char *name, ulong value, char type)
 {
+	if (XEN_HYPER_MODE() && STREQ(name, "__per_cpu_shift"))
+		return TRUE;
+
 	if (CRASHDEBUG(8) && name && strlen(name))
 		fprintf(fp, "%08lx %s\n", value, name);
 
--- a/x86_64.c
+++ b/x86_64.c
@@ -1989,6 +1989,9 @@ x86_64_verify_symbol(const char *name, u
 	if (!name || !strlen(name))
 		return FALSE;
 
+	if (XEN_HYPER_MODE() && STREQ(name, "__per_cpu_shift"))
+		return TRUE;
+
 	if (!(machdep->flags & KSYMS_START)) {
 		if (STREQ(name, "_text") || STREQ(name, "_stext")) {
 			machdep->flags |= KSYMS_START;