Re: [Xen-devel] [PATCH] x86/domctl: Don't pause the whole domain if only getting vcpu state

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx> 09/18/17 7:05 PM >>>
>On 09/18/2017 06:35 PM, Jan Beulich wrote:
>>>>> On 12.09.17 at 15:53, <aisaila@xxxxxxxxxxxxxxx> wrote:
>>> --- a/xen/arch/x86/domctl.c
>>> +++ b/xen/arch/x86/domctl.c
>>> @@ -625,6 +625,26 @@ long arch_do_domctl(
>>>               !is_hvm_domain(d) )
>>>              break;
>>>  
>>> +        if ( domctl->u.hvmcontext_partial.type == HVM_SAVE_CODE(CPU) &&
>>> +             domctl->u.hvmcontext_partial.instance < d->max_vcpus )
>> 
>> I have to admit that I'm not in favor of such special casing, even
>> less so without any code comment saying why this is so special.
>> What if someone else wanted some other piece of vCPU state
>> without pausing the entire domain? Wouldn't it be possible to
>> generalize this to cover all such state elements?
>
>There's no reason why all the other cases where this would the possible
>shouldn't be optimized. What has made this one stand out for us is that
>we're using it a lot with introspection, and the optimization counts.
>
>But judging by the code reorganization (the addition of
>hvm_save_one_cpu_ctxt()), the changes would need to be done on a
>one-by-one case anyway (different queries may require different ways of
>chaging the code).

But this function addition is precisely what I'd like to avoid in favor of
an extension to the existing mechanism using the registered function
pointers.

Jan


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