Date:	Tue, 26 Aug 2008 13:09:15 -0300
From:	"Luiz Fernando N. Capitulino" <lcapitulino@...driva.com.br>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Gerhard Brauer <gerhard.brauer@....de>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.{26.2,27-rc} oops on virtualbox

Em Tue, 26 Aug 2008 10:53:38 -0400
Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> escreveu:

| * Gerhard Brauer (gerhard.brauer@....de) wrote:
| > On Fri, Aug 22, 2008 at 02:08:13PM -0700, H. Peter Anvin wrote:
| > > Luiz Fernando N. Capitulino wrote:
| > >>
| > >>  I have asked Mandriva and Ubuntu users to test this and all of
| > >> them so far are saying that noreplace-paravirt works.
| > >>
| > >>  It makes the system slower, but it works.
| > >>
| > >
| > > Yes, the big issue is exactly what VirtualBox screws up in this matter,  
| > > how to detect it, and how to work around it.
| > >
| > > It's pretty clear it's a VirtualBox f*ckup at this point, but the failure 
| > > mechanism isn't at all obvious and so far the workaround is elusive.
| > >
| > > I'm strongly suspect this is a VirtualBox tcache management failure, but  
| > > that doesn't help the situation without knowing how it happens.
| > 
| > On Archlinux we have the same problem. We have a bugreport here:
| > http://bugs.archlinux.org/task/11141
| > 
| > Myself test it with a LiveCD/Install-ISO which has 2.6.26 as install
| > kernel. We have the guest oops on virtualbox-ose, virtualbox-sun and both on
| > i686 or x86_64 hosts.
| > 
| > Some things i noticed:
| > - The system boots always when i either enable VT-x in guest settings or
| >   disable acpi and run the guest with acpi=off.
| > - The oops occurs always on (disk)-io, no matter which file system i
| >   use.
| > - When the oops has occured and the guest has to close and restart then,
| >   if i don't use VT-x or acpi=off, i always get an oops directly when
| >   initrd/kernel is starting. Last screen message before the oops then is
| >   "Freeing SMP alternatives".
| > 
| > Here is also an archive with guest dmesg and messages.log from such an
| > oops when heavy disk io leads to the oops:
| > http://bugs.archlinux.org/task/11141?getfile=2445
| > 
| 
| Hrm, can you try this ?
| 
| 1 - Make sure you kernel is not CONFIG_DEBUG_RODATA

"""
$ grep CONFIG_DEBUG_RODATA .config
# CONFIG_DEBUG_RODATA is not set
$
"""

| 2 - Change the whole text_poke implementation in
| arch/x86/kernel/alternative.c to this :
| 
| void *__kprobes text_poke(void *addr, const void *opcode, size_t len)
| {
|   return text_poke_early(addr, opcode, len);
| }
| 
| If this works, I suspect that the problem comes from a vmap/vunmap
| problem. If it still fails, the problem would likely come from a race
| with interrupt disabling probably due to missing data/instruction cache
| flush.

 I still get the oops with this change. :((

| Then, after having tested (2), try this on top of it :
| 
| In arch/x86/kernel/alternative.c, alternatives_smp_switch()
| 
| Add   unsigned long flags;
| Change 
| spin_lock -> spin_lock_irqsave(&smp_alt, flags);
| spin_unlock(&smp_alt); -> spin_unlock_irqrestore(&smp_alt, flags);
| 
| This will help testing if there is a problem with interrupts coming
| shortly after the modification. If it fixes the problem, my guess is
| that we should flush the instruction cache (and maybe the data cache ?)
| in text_poke and text_poke early when interrupts are off.

 By 'on top of it' you mean I should make these changes with the
text_poke() version above right?

 By the way, I have added a comment in the virtualbox's bugzilla
pointing out this thread but no feedback from them so far.

-- 
Luiz Fernando N. Capitulino
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