Re: [Xen-devel] [XEN PATCH v1 1/1] x86/vm_event: add fast single step

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Tue, Dec 17, 2019 at 7:48 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 17/12/2019 14:40, Sergey Kovalev wrote:
> > On break point event eight context switches occures.
> >
> > With fast single step it is possible to shorten path for two context
> > switches
> > and gain 35% spead-up.
> >
> > Was tested on Debian branch of Xen 4.12. See at:
> > https://github.com/skvl/xen/tree/debian/knorrie/4.12/fast-singlestep
> >
> > Rebased on master:
> > https://github.com/skvl/xen/tree/fast-singlestep
> >
> > Signed-off-by: Sergey Kovalev <valor@xxxxxxx>
>
> 35% looks like a good number, but what is "fast single step"?  All this
> appears to be is plumbing for to cause an altp2m switch on single step.

Yes, a better explanation would be much needed here and I'm not 100%
sure it correctly implements what I think it tries to.

This is my interpretation of what the idea is: when using DRAKVUF (or
another system using altp2m with shadow pages similar to what I
describe in 
https://xenproject.org/2016/04/13/stealthy-monitoring-with-xen-altp2m),
after a breakpoint is hit the system switches to the default
unrestricted altp2m view with singlestep enabled. When the singlestep
traps to Xen another vm_event is sent to the monitor agent, which then
normally disables singlestepping and switches the altp2m view back to
the restricted view. This patch looks like its short-circuiting that
last part so that it doesn't need to send the vm_event out for the
singlestep event and should switch back to the restricted view in Xen
automatically. It's a nice optimization. But what seems to be missing
is the altp2m switch itself.

Tamas

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