linux-kernel - Re: [PATCH] kmemcheck: SMP support

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Date:	Fri, 23 May 2008 19:32:52 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Jan Kiszka" <jan.kiszka@....de>
Cc:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Pekka Enberg" <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kmemcheck: SMP support

On Fri, May 23, 2008 at 7:12 PM, Jan Kiszka <jan.kiszka@....de> wrote:
> Vegard Nossum wrote:
>> On Fri, May 23, 2008 at 5:40 PM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>> Vegard Nossum wrote:
>>>> This works on real hw, but not on qemu. It seems to get stuck waiting for
>>>> one
>>>> of the atomic values to change. Don't know why yet, it might just be yet
>>>> another bug in qemu... (we've hit at least two of them so far. And they
>>>> were
>>>> real bugs too.)
>>>>
>>> I've noticed that qemu mis-reports the eip of cmpxchg if it faults (it
>>> reports the eip of the start of the basic block, I think).  Does that match
>>> what you're seeing?
>>
>> You mean the EIP that gets pushed on the stack for the page fault?
>> (That would be bad news for kmemcheck. I suppose the rest of the
>> kernel never page faults on cmpxchg addresses?)
>>
>> Or do you mean the EIP that shows up in gdb?
>>
>> But no, it seems to be unrelated. What I hit so far were (in 0.9.0):
>>
>> 1. qemu doesn't set the single-stepping flag of DR6 on single-step
>> debug exceptions.
>> 2. qemu triggers int 0 (divide error) instead of int 2 on NMI IPIs.
>>
>> But both of these were fixed in the latest 0.9.1.
>
> I guess you mean trunk - NMI IPIs didn't came with "old" 0.9.1.

Are you sure? It does in fact deliver the NMI IPI as far as I can see
and I am running from a qemu-0.9.1.tar.gz... E.g. for "-smp 3" on this
0.9.1 qemu:

(first number is smp_processor_id())

[0 pause all] <-- in page fault handler
[1 paused] <-- in nmi handler
[2 paused]
[0 resume all] <-- in debug exception handler
[2 resuming, paused = 1] <-- still in nmi handler, now exiting
[1 resuming, paused = 0]

But maybe I should try the trunk and see if that fixes the problem I was seeing!

>
>>
>> I don't yet know if what I'm hitting now is really an error with qemu.
>> But I usually trust the real hardware more :-)
>
> Try KVM as well. It is, of course, must faster than QEMU, and it comes
> with true SMP (given you have a SMP host). With in-kernel irqchip
> (that's default), KVM now also supports NMI IPIs. And debug registers
> should be fine with my latest patch.
>
> I'm currently trying to get debug support straight for upstream KVM and,
> where also required, QEMU. SMP debugging is a common issue, but already
> usable with KVM. So testers are welcome, an overview on required patches
> can be provided.
>

Hm. Doesn't KVM require special hardware? I have just a cheap laptop
(Pentium Dual-Core) and I doubt I will be able to run it... :-(

Thanks for the tip, though!


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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