linux-kernel - Re: Another preempt folding issue?

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Message-ID: <52FB5669.7090506@canonical.com>
Date:	Wed, 12 Feb 2014 12:09:29 +0100
From:	Stefan Bader <stefan.bader@...onical.com>
To:	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>
CC:	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 12.02.2014 11:40, Borislav Petkov wrote:
> On Wed, Feb 12, 2014 at 11:37:13AM +0100, Peter Zijlstra wrote:
>>> Another reporter also saw this on an AMD and said it could not be reproduced on
>>> the same hardware and the same software versions when using 64bit instead of 32.
>>>
>>> In my case on a 32bit installation I will see this on every invocation of
>>
>> Curious.. so its both AMD and Intel.
>>
>> That makes funny hardware less likely... but how can the vmexit loose an
>> interrupt like this.
> 
> Also what I'm wondering about and what's not clear from Stefan's reply
> is whether this is purely a 32-bit issue, i.e. a 32-bit host running a
> 64-bit qemu running a 32-bit iso or what is it?
> 
> Or do we have reports for both 32-bit and 64-bit hosts?

Only for 32bit hosts. I have not tested a 64bit kernel plus 32bit userspace,
though. But 64bit kernel and 64bit user-space seemed ok. And one detail I should
add is that while the reproduction is possible in all attempts, the timing can
vary. In some of the cases this was even before running a Linux kernel in the guest.

Something else here I run a kernel with CONFIG_PREEMPT not set and NR_CPUS
limited to 8 (for the 32bit kernel). So the default apic driver is used. Since
default_send_IPI_mask_logical is only used from there, I assume the trace you
got does the same. Maybe something there is wrong which would explain why we
only see it on 32bit hosts.

> 
> Thanks.
> 

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