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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

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).



Thanks,
Razvan

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