Re: [Xen-devel] AMD support of hvm_vcpu.single_step

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Wed, 11 Jun 2014 19:46:12 -0400
Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:

> On 06/11/2014 07:04 PM, Mukesh Rathor wrote:
> > On Wed, 11 Jun 2014 18:55:53 -0400
> > Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
> >
> >> On 06/11/2014 06:32 PM, Mukesh Rathor wrote:
> >>> On Wed, 11 Jun 2014 11:56:15 -0400
> >>> Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
> >>>
> >>>> On 06/10/2014 05:49 PM, Mukesh Rathor wrote:
> >>>>> Hi AMD folks,
> >>>>>
> >>>>> I am unable to see support of single step on amd platform. On
> >>>>> intel, hvm_vcpu.single_step will cause MTF to be set in
> >>>>> vmx_intr_assist:
> >>>>>
> >>>>>        /* Block event injection when single step with MTF. */
> >>>>>        if ( unlikely(v->arch.hvm_vcpu.single_step) )
> >>>>>        {
> >>>>>            v->arch.hvm_vmx.exec_control |=
> >>>>> CPU_BASED_MONITOR_TRAP_FLAG; vmx_update_cpu_exec_control(v);
> >>>>>            return;
> >>>>>        }
> >>>>>
> >>>>> I don't see equivalent on amd?
> >>>> I think HW performs TF bit manipulations that your patch is
> >>>> suggesting (per 13.1.4 of APMv2)
> >>> It only clears it, not set it. Patch on the way.
> >>
> >> Does it need to be set? My understanding is that it is supposed to
> >> be clear in the handler (in the guest) but set on the exception's
> >> stack so when the guest does the IRET it loads TF=1.
> > If a debugger wants to single step, it needs to set the TF. So this
> > is for the debugger path. The hardware clears it upon execution of
> > next instruction, and generates #DB.
> 
> The patch seems to be setting TF when returning from #DB intercept, 
> right? Won't this result in another #DB on the first instruction of 
> guest's #DB handler?

Nop, we are setting the TF in the resume path, returning from whatever
intercept it was. It could have been anything. If an external debugger set the
single_step flag during this time, then just like vmx, svm must honor that 
and do what it takes, in this case setting the TF (vmx sets MTF) when
it's resuming the guest. 

thanks,
Mukesh


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