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

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Date:	Mon, 16 Mar 2009 12:04:10 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	akataria@...are.com, the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: VMI broken on tip/master...


On Sun, 15 Mar 2009, Peter Zijlstra wrote:
> 
> 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.

It should only blow up if a guest is using tracing, and the code to call
the hypervisor is also being traced.

> 
> 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? :-)

I have noticed that some paravirt ops calls (like this 
raw_local_irq_disable) does not get inlined. It sometimes gets made into a 
function.  This would cause raw_local_irq_disable to actually be traced!

One answer is to use "always_inline" or I can dig out a patch that makes 
inline also include "notrace".

-- Steve

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