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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 02/04/2014 11:01 AM, Jan Beulich wrote:
>>>> On 04.02.14 at 16:53, Boris Ostrovsky <boris.ostrovsky@oracle.com> wrote:
>> On 02/04/2014 06:31 AM, Jan Beulich wrote:
>>>>>> On 21.01.14 at 20:08, Boris Ostrovsky <boris.ostrovsky@oracle.com> 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