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Date:	Sat, 05 Apr 2008 10:36:50 +0300
From:	Avi Kivity <>
To:	Pekka Paalanen <>
CC:, Ingo Molnar <>,
	Christoph Hellwig <>,
	Arjan van de Ven <>,
	Pavel Roskin <>,
	Steven Rostedt <>,
	Peter Zijlstra <>,,
Subject: Re: mmiotrace bug: recursive probe hit

Pekka Paalanen wrote:
> On Sun, 30 Mar 2008 20:26:08 +0300
> Pekka Paalanen <> wrote:
>> C) Vegard mentioned something about per-cpu page tables for kmemcheck.
>> This would be the ultimate solution, because it would solve two problems:
>> - recursive probe hits
>> - missed events due to another cpu disarming the page for single stepping
>> Would it be possible to have a single temporary per-cpu pte?
>> I understood kmemcheck has similar issues. Of course, one could force the
>> system down to a single running CPU, but that feels nasty.
> One more idea:
> The catch is the instruction emulation. I see KVM has some emulation code,
> but I cannot understand it without a deep study that would take me weeks.
> Is that general enough to be used, or could it be generalized?
> Mmiotrace, apart from executing the instruction with a modified address,
> would need to extract the type of IO memory access, width and the data
> read/written. And since it is dealing with IO memory, the emulation
> should be very careful to access the hardware exactly like the original
> instruction would have.
> Maybe also kmemcheck could use this approach, since the current approach
> is very much like in mmiotrace: #pf, show page, single step, #db trap,
> hide page.
> Are there other x86(_64) instruction emulation facilities in the kernel
> I might use?
> Or, if the emulation cannot be used, what would it take to make at least
> instruction decoding general enough so that mmiotrace could use that instead
> of its own decoding?
> I fear modifying KVM emulation code is a too heavy job for me personally.

It should not be too difficult to modify x86_emulate.c to do everything 
through a function vector.  However there is a simpler (for you) 
solution: run the driver-to-be-reverse-engineered in a kvm guest, and 
modify kvm userspace to log accesses to mmio regions.  This requires the 
not-yet-merged pci passthrough support.  You can reverse engineer 
Windows drivers with this as well.

This won't work for kmemcheck smp though.

Any sufficiently difficult bug is indistinguishable from a feature.

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