+++ This bug was initially created as a clone of Bug #425471 +++

Description of problem:
My test environment is the following:

dom0: RHEL5.2 i686 kernel, 2.6.18-59.el5xen, 8GB memory, 4 physical CPUs, AMD Rev. F
domU: RHEL5.2 i686 kernel, 2.6.18-59.el5xen, 1GB memory, 4 vCPUs

Inside the domU, I run a kernel compile in a loop with "make -j5".
On the dom0, I run an "xm save <dom> /var/lib/xen/save/<dom>-save ; xm restore
/var/lib/xen/save/<dom>-save" in a loop.

After about 100 iterations of the above save/restore loop, the save will hang,
with the following messages in the domU console:

do_IRQ: stack overflow: 488
 [<c0406e3c>] <1>BUG: unable to handle kernel paging request at virtual address
35416001

(full stack trace is attached).  Note that with the same load, I do not see this
problem on x86_64.

-- Additional comment from clalance@redhat.com on 2007-12-14 16:24 EST --
Created an attachment (id=289491)
Full stack trace when the crash occurs


-- Additional comment from ijc@hellion.org.uk on 2008-01-18 11:36 EST --
I think we might be seeing similar on 2.6.18-53.1.4.el5 (plus fixes for the bugs
which we have reported against 5.1) when doing migration under load.

Unfortunately either the dump_trace() code is buggy or our attempt to call it
causes us to really overflow so our backtrace is less useful than yours. I will
attach it anyway.

-- Additional comment from ijc@hellion.org.uk on 2008-01-18 11:37 EST --
Created an attachment (id=292173)
Another similar stack trace


-- Additional comment from ddutile@redhat.com on 2008-01-30 15:46 EST --
Created an attachment (id=293483)
vcpu-init stack overflow patch


-- Additional comment from ddutile@redhat.com on 2008-01-30 15:47 EST --
Ian,

Can you give the patch in #4 a try in your test scenario.
Seems to work for our test case (so far; test look exceeding 140 now; will let
it continue to run overnight...)

- Don

-- Additional comment from clalance@redhat.com on 2008-01-30 16:35 EST --
The patch attached here looks like it will work, because __cpu_up() is
serialized in the common kernel/cpu.c code.  That being said, it looks like
upstream LKML xen went a different way, and just used some kzalloc() memory in
arch/x86/xen/smp.c (their equivalent to drivers/xen/core/smpboot.c).  There's no
real changeset to point to; it was merged into Linus's tree that way.  Anyway,
just thought I'd point this out, since this way has some additional upstream
testing behind it.

Chris Lalancette

-- Additional comment from ddutile@redhat.com on 2008-01-30 17:09 EST --
Created an attachment (id=293499)
Updated/new patch based on LKML version; note: used kmalloc & memset so it
works on older linux versions.


-- Additional comment from ddutile@redhat.com on 2008-01-30 17:25 EST --
Created an attachment (id=293504)
oops... last patch had incorrect memset;  caught it before starting overnight
test..... nothing like exchanging one stack overflow for a stack trashing! ;-)


-- Additional comment from ijc@hellion.org.uk on 2008-01-31 09:20 EST --
Latest patch crashes on boot for me.

I thought at first it was because:
  BUG_ON(HYPERVISOR_vcpu_op(VCPUOP_initialise, cpu, &ctxt));
should be
  BUG_ON(HYPERVISOR_vcpu_op(VCPUOP_initialise, cpu, ctxt));

But changing that didn't fix it.

Also I think the "memset(&ctxt->fpu_ctxt, 0, sizeof(ctxt->fpu_ctxt));" is
unecessary since this will be clear as part of the memset over the entire ctxt.
Shouldn't jurt to do it though.

-- Additional comment from ddutile@redhat.com on 2008-01-31 10:05 EST --
Created an attachment (id=293587)
a fix to the last fix!

Ian,

yes, the hypervisor call bugs out.... and this piece of code isn't much
better either:
	 kmalloc(sizeof(*ctxt), GFP_KERNEL);  <-- ctxt isn't set equal to
return!

turns out i did an overnight test w/the static, and that ran for over 600
iterations, so the concept of reducing stack size definitely solves the
problem.
Now, to implement the patch correctly! ;-)

So, sorry for the late day mess up. 
Try this updated patch.  if that runs long today, I'll post it on xen-devel,
as well as dupe it for rhel4.

- Don