linux-kernel - Re: VMI broken on tip/master...

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Date:	Sun, 15 Mar 2009 00:20:00 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	akataria@...are.com
Cc:	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: VMI broken on tip/master...

On Fri, 2009-03-13 at 10:56 -0700, 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 ?

Looking at arch/x86/include/asm/paravirt.h (god I wish paravirt would go
away, not only does it screw over ctags, it also hurts my brain), it
appears its playing icky games with primitives like
raw_local_irq_disable():

static inline void raw_local_irq_disable(void)
{
        asm volatile(paravirt_alt(PARAVIRT_CALL)
                     :
                     : paravirt_type(pv_irq_ops.irq_disable),
                       paravirt_clobber(CLBR_EAX)
                     : "memory", "eax", "cc");
}

So what was supposed to be a simple op, now gets expanded into god knows
what, and might lead to tracer recursion or something.

Maybe a simple notrace annotation somewhere in that paravirt code is all
it takes, who knows.

Steve, you've been known to work on virt stuff too, happen to have a
bright idea? :-)

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