Re: [Publicity] Stealthy monitoring with Xen altp2m

["Lengyel, Tamas" <tlengyel@xxxxxxxxxxx>]

Hi Lars,

if you think this would be an appropriate post for the Xen blog I would be happy to make another round of editing to address Razvan's comments. Just let me know!

Thanks,

Tamas

On Thu, Jan 7, 2016 at 11:25 AM, Lars Kurth <lars.kurth.xen@xxxxxxxxx> wrote:

Hi folks,

I just saw this. Given that there were some technical comments, does it make sense for Tamas to incorporate these and I and then I and Sarah can have a look to make it more punchy?

Lars

On 4 Jan 2016, at 20:39, Lengyel, Tamas <tlengyel@xxxxxxxxxxx> wrote:

> >> This is definitely not a very detailed analysis of what's happening, but
> >> I would think that the latter process takes at least as much time as the
> >> former. In other words, I don't see how the altp2m case improves
> >> performance.
> >
> > I don't think it does either.
>
> But then I have misread your statement that "However, this solution
> (while supported) is not particularly ideal as it still creates
> significant performance overhead." - I've read it to mean that the
> significant performance overhead you're talking about applies to the
> emulation case, as opposed to the better altp2m way - which, as it turns
> out, neither of us believes.
>

Right, what I meant there is execution tracing with EPT is not optimal because of the extra overhead of the unrelated instruction fetch violations on the page. The performance gain is achieved with the breakpoint trick in DRAKVUF, not specific to altp2m (only triggers at specific points instead of the whole page). It only applies for excecution tracing, but in my use-case that had been the major goal and thus the main source of overhead. Here I highlight that this lighter monitoring can still be used on multi-vCPU systems safely with altp2m by doing the shadow copy/remapping method. I can try to rework this part a bit to avoid the confusion.

Cheers,
Tamas

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