Re: Pausing / unpausing a single domain VCPU via libxc

[Razvan Cojocaru <rcojocaru@bitdefender.com>]

On 01/26/2017 06:00 PM, Dario Faggioli wrote:
> On Thu, 2017-01-26 at 17:42 +0200, Razvan Cojocaru wrote:
>> On 01/26/2017 05:14 PM, Dario Faggioli wrote:
>>> You mean you'd want to implement xc_vcpu_pause() by means of
>>> the XEN_DOMCTL_gdbsx_pausevcpu?
>>>
>>> What's the use case for that, and does it fit with the
>>> implementation
>>> of said hypercall (which, e.g., requires that the domain is already
>>> paused)?
>>
>> Not necessarily implement xc_vcpu_pause() by means of
>> XEN_DOMCTL_gdbsx_pausevcpu, it's just that from a design perspective
>> this seems rather specialised - the domctl name suggests that this is
>> only useful for debugging, and it also hardcodes an implementation
>> for
>> calling that hypercall in xg_main.c, whereas pausing a single VCPU is
>> a
>> generic operation that other clients, as well as the debugger, might
>> find worthwhile to do.
>>
> And, in principle, I agree that it would make sense to have a generic
> vCPU pause/unpause API.
> 
> Outside of principle --a.k.a., in practise-- I'm not sure whether it
> really makes sense, and whether it would actually work.
> 
> I'm thinking of how happy a guest would be to be running with one of
> it's (v)CPU frozen, and am wondering whether this could cause it to
> panic, or behave funnily.
> 
> But I haven't tried, of course.
> 
>> Does XEN_DOMCTL_gdbsx_pausevcpu require that the whole domain is
>> paused
>> (for longer that the duration of the actual hypercall)?
>>
> AFAICT, it does. See the implementation of the hypercall:
> 
>         if ( !d->controller_pause_count )
>             break;
>         ret = -EINVAL;
>         if ( domctl->u.gdbsx_pauseunp_vcpu.vcpu >= d->max_vcpus ||
>              (v = d->vcpu[domctl->u.gdbsx_pauseunp_vcpu.vcpu]) == NULL )
>             break;
>         ret = vcpu_pause_by_systemcontroller(v);
> 
> See commit 680d79f10 ("x86/gdbsx: invert preconditions for
> XEN_DOMCTL_gdbsx_{,un}pausevcpu hypercalls":
> 
>     Revert back to how it was originally, i.e. the XEN_DOMCTL_gdbsx_{,un}pausevcpu
>     hypercalls are only valid for a domain already paused by the system controller.
> 
> And see how it's used, e.g., as described in the comment above
> xg_step() (or even in its actual code):
> 
> /*
>  * Single step the given vcpu. This is achieved by pausing all but given vcpus,
>  * setting the TF flag, let the domain run and pause, unpause all vcpus, and
>  * clear TF flag on given vcpu.
>  * Returns: 0 success
>  */

Right. Thank you for clearing that up!


Thanks,
Razvan

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