Re: [Xen-devel] [PATCH v4 13/17] x86/VPMU: Add privileged PMU mode

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/04/2014 11:01 AM, Jan Beulich wrote:
> > > > On 04.02.14 at 16:53, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
> > On 02/04/2014 06:31 AM, Jan Beulich wrote:
> > > > > > On 21.01.14 at 20:08, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
> > > > +            gregs = &v->arch.vpmu.xenpmu_data->pmu.r.regs;
> > > > +            gregs->cs = cs;
> > > And now you store a NUL selector (i.e. just the RPL bits) into the
> > > output field?
> > > >            }
> > > > -        else if ( !is_control_domain(current->domain) &&
> > > > -                 !is_idle_vcpu(current) )
> > > > +        else
> > > >            {
> > > > -            /* PV guest */
> > > > +            /* HVM guest */
> > > > +            struct segment_register cs;
> > > > +
> > > >                gregs = guest_cpu_user_regs();
> > > >                memcpy(&v->arch.vpmu.xenpmu_data->pmu.r.regs,
> > > >                       gregs, sizeof(struct cpu_user_regs));
> > > > +
> > > > +            hvm_get_segment_register(current, x86_seg_cs, &cs);
> > > > +            gregs = &v->arch.vpmu.xenpmu_data->pmu.r.regs;
> > > > +            gregs->cs = cs.attr.fields.dpl;
> > > And here too? If that's intended, a code comment is a must.
> > This is HVM-only path, PVH or PV don't go here so cs should be valid.
> Isn't the reply of mine a few lines up in PV code? And why would
> the selector being wrong for HVM be okay?

This clause is for privileged profiling: we are in PV clause even though 
the interrupt is taken by an HVM guest.
The diff is somewhat difficult to follow so here is the flow:

   // in privileged mode 'v' is dom0's CPU.
   if ( !is_hvm_domain(v->domain) || (vpmu_mode & XENPMU_MODE_PRIV) )
   {
        if ( !is_hvm_domain(current->domain) )
        {
             // either PV (including dom0) or Xen is interrupted
        }
        else
        {
             // This is the clause we are discussing. 'current' is HVM
             hvm_get_segment_register(current, x86_seg_cs, &cs);
        }
        send_guest_vcpu_virq(v, VIRQ_XENPMU);
        return 1;
   }

So I think CS should be correct for the guest, no?


-boris


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