linux-kernel - Re: [PATCH] kvm: Fix build warnings

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Message-ID: <20110531090219.GA26762@liondog.tnic>
Date:	Tue, 31 May 2011 11:02:19 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Avi Kivity <avi@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Marcelo Tosatti <mtosatti@...hat.com>,
	kvm@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
Subject: Re: [PATCH] kvm: Fix build warnings

On Tue, May 31, 2011 at 11:20:55AM +0300, Avi Kivity wrote:
> On 05/31/2011 10:38 AM, Ingo Molnar wrote:
> >* Borislav Petkov<bp@...en8.de>  wrote:
> >
> >>  +++ b/arch/x86/kvm/paging_tmpl.h
> >>  @@ -121,7 +121,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
> >>   				gva_t addr, u32 access)
> >>   {
> >>   	pt_element_t pte;
> >>  -	pt_element_t __user *ptep_user;
> >>  +	pt_element_t __user *uninitialized_var(ptep_user);
> >
> >Note that doing this is actually actively dangerous for two reasons.
> >
> >
> 
> <snip lots of good advice>
> 
> >Please fix it instead.
> 
> s/instead/in addition/; while all those changes are good, they are
> much too large for 3.0.  Let's push the simple fix for 3.0 and queue
> the bigger refactoring to 3.1.

Just to clarify: Hell, it is _not_ I who's fixing this! Virtualization
folks are crazy anyway and I'm not touching their code except for
trivial fixes :-).

The story: I saw the humongous function and being lazier than Ingo, I
just wanted to shut up the warning. Knowing that uninitialized_var()
is a dangerous thing to use, I asked whether people who know the code
can guarantee that ptep_user is not going to be used uninitialized and
Takuya confirmed.

But yes, it'll be much better if kvm people could take a good hard look
at it and try to simplify it. Also, while they're at it, I'd suggest
they actually trace whether that unlikely() annotation actually brings
any performance speedup - if it doesn't, out the door with it and here's
more simplification right there.

Thanks.

-- 
Regards/Gruss,
    Boris.
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