Re: [-mm patch] enforce noreplace-smp in alternative_instructions()

[Frederik Deweerdt <deweerdt@free.fr>]

[Added Gerd Hoffman and Rusty Russel to cc]
On Thu, Aug 23, 2007 at 11:46:52PM -0700, Jeremy Fitzhardinge wrote:
> Frederik Deweerdt wrote:
> > That means that even when you specify noreplace_smp, some replacing
> > takes place anyway. One of the consequences, besides noreplace_smp not
> > working as expected, is that lguest crashes when you feed it an SMP kernel
> > (I suspect that you can not replace alternatives for smp _and_ paravirt).
> >   
> 
> No, that should be fine.  Why does lguest crash?
It dies with:
[    0.131000] SMP alternatives: switching to UP code
lguest: bad stack page 0xc057a000

I added a dump_stack on the Host, which gives:
[124320.090946]  [<c01052f8>] dump_trace+0x65/0x1de
[124320.090956]  [<c010548b>] show_trace_log_lvl+0x1a/0x2f
[124320.090970]  [<c0105ea4>] show_trace+0x12/0x14
[124320.090975]  [<c0105fcd>] dump_stack+0x16/0x18
[124320.090980]  [<f888032c>] pin_page+0x5f/0xa3 [lg]
[124320.090993]  [<f8880654>] pin_stack_pages+0x3a/0x4a [lg]
[124320.091004]  [<f888007e>] guest_pagetable_clear_all+0x12/0x15 [lg]
[124320.091013]  [<f887f81a>] do_hcall+0xb1/0x1cb [lg]
[124320.091021]  [<f887fbbe>] do_hypercalls+0x28a/0x2a0 [lg]
[124320.091029]  [<f887f2a2>] run_guest+0x24/0x492 [lg]
[124320.091037]  [<f8881b48>] read+0x83/0x8f [lg]
[124320.091048]  [<c0175a77>] vfs_read+0x8e/0x117
[124320.091054]  [<c0175e99>] sys_read+0x3d/0x61
[124320.091059]  [<c0104166>] sysenter_past_esp+0x6b/0xb5
[124320.091065]  [<ffffe410>] 0xffffe410
[124320.091069]  =======================

Now, the "SMP alternatives: switching to UP code" message made me wonder
if it had anything to do with the alternatives, so I tried disabling
the switch by passing noreplace_smp...
... But the message was displayes anyway (and the smp_locks section
freed), because the check my patch adds is not made.
With the patch, I can boot lguest with an SMP kernel if I pass
noreplace_smp.
> 
> > I agree, but I don't think it is doable (alt_smp_once comes to mind). I'll
> > double check however.
> 
> Hm.  Is alt_smp_once useful?

I can't figure what the use case is, debugging set aside,
but there are places (eg xen, __cpu_die) in the kernel calling
alternatives_smp_switch(1) at runtime.  Passing smp-alt-once will prevent
the switch.

Regards,
Frederik
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