Re: [Xen-devel] Mem_event API and MEM_EVENT_REASON_SINGLESTEP

[AP <apxeng@xxxxxxxxx>]

On Fri, Nov 30, 2012 at 1:04 PM, Razvan Cojocaru <rzvncj@xxxxxxxxx> wrote:
>> Not sure what the difference is between first write operation and
>> first write mem_event.
>
> The difference is this: a write operation will only trigger a write
> event for a page marked rx. So if we're in single step mode and a write
> happens on a page marked rwx, this will not trigger a write mem_event.
>
>>> My scenario is this: I'd like to mark _all_ of the domain's pages rw,
>>> then when I get a write mem_event, mark only one page rwx, allow that
>>> write (single stepping), then mark that one page rw again. This would
>>
>> Do you mean rx? If you mark a page rw you will not get a write mem_event for 
>> it.
>
> Yes, I meant rx. Sorry (it's late where I'm writing from).
>
>> You could track the EIP and GFN and if you single step more than one
>> instruction you can mark the page RX and stop single stepping. You
>> will get control again on the next write. Though if there are jmp,
>> things could get tricky.
>
> So if I understand you correctly, simply single-stepping for only the
> duration of one MEM_EVENT_REASON_SINGLESTEP, which should be the write
> operation (ignoring the gfn/gla fields of the mem_event), should do the
> trick?

That is correct. Let me know how it goes.

Cheers,
AP

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