Re: VMI broken on tip/master...

From: Alok Kataria
Date: Mon Mar 16 2009 - 18:42:20 EST


On Sat, 2009-03-14 at 16:45 -0700, Jeremy Fitzhardinge wrote:
> Alok Kataria wrote:
> > Hi Peter,
> >
> > I was seeing a early fault when running tip/master with VMI enabled on
> > VMware platform.
> > This early fault was in the vmi_patch code where we are applying
> > paravirt_alternatives. After some trials i noticed that this is
> > reproducible only with CONFIG_TRACING. With that disabled my VM boots
> > again.
> >
> > I started a git bisect after that, and git pointed to this as the bad
> > commit
> >
> > commit 6cc3c6e12bb039047974ad2e7e2d46d15a1b762f
> > trace_clock: fix preemption bug
> >
> > I then reverted that commit from tip/master and the system did boot.
> > But I fail to understand how this simple patch would be causing things
> > to fail in VMI. Any ideas ?
> >
>
> Nope. My first guess is that this is a misbisection, but the fact that
> reverting helps tends to undermine that diagnosis.
>
> What crash are you seeing? What kind of fault? At what instruction?

It being a early fault, nothing is printed on the console the system
just stays stuck in this early_fault code in arch/x86/kernel/head_32.S

I don't understand why is this not printing anything at this early fault
though, the system just enters the hlt_loop and stays there.

> Doing what? It's a bit hard to tell what you're actually seeing.
I did some more debugging and I think i know what the problem is
The objdump for trace_clock_local looks like this

c1070c24 <trace_clock_local>:
c1070c24: 55 push %ebp
c1070c25: 89 e5 mov %esp,%ebp
c1070c27: 53 push %ebx
c1070c28: 83 ec 08 sub $0x8,%esp
c1070c2b: ff 15 5c 87 3d c1 call *0xc13d875c
c1070c31: ba 64 87 3d c1 mov $0xc13d8764,%edx
c1070c36: 89 45 f4 mov %eax,0xfffffff4(%ebp)
c1070c39: ff 12 call *(%edx) <<=====*
c1070c3b: e8 cd 68 f9 ff call c100750d <sched_clock>
c1070c40: 89 c1 mov %eax,%ecx
c1070c42: 8b 45 f4 mov 0xfffffff4(%ebp),%eax
c1070c45: ff 15 60 87 3d c1 call *0xc13d8760

Notice instruction on c1070c39 we have "call *(edx)",
edx was just loaded with the address for the paravirt call.
when we try to replace that to a call to vmi specific function, maybe we
hit a BUG_ON(len < 5) in vmi's patch_internal code, because now the
instruction length is less than 5.

Is there is a way to get GCC to not do such fancy tricks, and instead do
a direct "call 0xc13d8764" ?

Thanks,
Alok
>
> Does reverting the text_poke() changes make a difference?
>
> J

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