linux-kernel - Re: Another preempt folding issue?

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Message-ID: <20140214173333.GA6953@pd.tnic>
Date:	Fri, 14 Feb 2014 18:33:33 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Stefan Bader <stefan.bader@...onical.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kvm@...r.kernel.org, Marcelo Tosatti <mtosatti@...hat.com>,
	MASAO TAKAHASHI <masao-takahashi@...no.co.jp>,
	Joerg Roedel <joro@...tes.org>
Subject: Re: Another preempt folding issue?

On Fri, Feb 14, 2014 at 06:02:32PM +0100, Stefan Bader wrote:
> Okaaay, I think I did what you asked. So yes, there is sse2 in the cpu info. And
> there is a mfence in the disassembly:

Btw, I just realized booting the kernel in the guest was a dumb idea,
because, doh, the guest is not baremetal. The only reliable thing we
can say is that sse2 is present and that MFENCE alternative replacement
works :)

But for simplicity's sake let's just assume the machine can do MFENCE
just fine and it gets replaced by the alternatives code.

Besides, if that weren't true, we'd have a whole lot of other problems
on those boxes.

> Thinking about it, I guess Peter is quite right saying that I likely
> will end on the patch that converted preempt_count to percpu.

Yeah, c2daa3bed53a81171cf8c1a36db798e82b91afe8 et al.

> One thing I likely should do is to reinstall the exact same laptop
> with 64bit kernel and userspace... maybe only 64bit kernel first...
> and make sure on my side that this does not show up on 64bit, too. I
> took the word of reporters for that (and the impression that otherwise
> many more people would have complained).

Yeah, that should be a prudent thing to do.

Also, Paolo and I were wondering whether you can trigger this thing
without kvm, i.e. virtualization involved... do you have any data on
that?

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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