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

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

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.

-Mukesh



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