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

[Sergey Kovalev <valor@xxxxxxx>]

On 17.12.2019 17:48, Andrew Cooper 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.
>
> ~Andrew
You are right. I should quoted "fast single step".

Original INT#3 path is like this (in PlangUML):
@startuml
VM->Xen : EXIT_REASON_EXCEPTION_NMI
Xen->LibVMI: request(VM_EVENT_REASON_SOFTWARE_BREAKPOINT)
LibVMI->Xen: response(singlestep | altp2m)
Xen->VM:
VM->Xen: EXIT_REASON_MONITOR_TRAP_FLAG
Xen->LibVMI: request(VM_EVENT_REASON_SINGLESTEP)
LibVMI->Xen: response(altp2m)
Xen->VM:
@enduml

With fast single step it looks like this:
@startuml
VM->Xen : EXIT_REASON_EXCEPTION_NMI
Xen->LibVMI: request(VM_EVENT_REASON_SOFTWARE_BREAKPOINT)
LibVMI->Xen: response(fast singlestep | altp2m)
Xen->VM:
VM->Xen: EXIT_REASON_MONITOR_TRAP_FLAG
Xen->Xen: fast singlestep
Xen->VM:
@enduml

So we just store altp2m index and switch to it on MTF.

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