Re: [Qemu-devel] [PATCH RESEND v2] i386/kvm: add support for KVM_CAP_X86_DISABLE_EXITS

[Paolo Bonzini]

On 19/04/2018 21:56, Eduardo Habkost wrote:
> On Thu, Apr 19, 2018 at 05:48:57PM +0200, Paolo Bonzini wrote:
>> On 17/04/2018 22:59, Eduardo Habkost wrote:
>>>> +        if (disable_exits) {
>>>> +            disable_exits &= (KVM_X86_DISABLE_EXITS_MWAIT |
>>>> +                              KVM_X86_DISABLE_EXITS_HLT |
>>>> +                              KVM_X86_DISABLE_EXITS_PAUSE);
>>>> +            if (env->user_features[FEAT_KVM] & KVM_PV_UNHALT) {
>>>> +                disable_exits &= ~KVM_X86_DISABLE_EXITS_HLT;
>>>> +            }
>>>
>>> In the future, if we decide to enable kvm-pv-unhalt by default,
>>> should "-cpu ...,kvm-hint-dedicated=on" disable kvm-pv-unhalt
>>> automatically, or should we require an explicit
>>> "kvm-hint-dedicated=on,kvm-pv-unhalt=off" option?
>>
>> It should be automatic.
>>
>>> For today's defaults, this patch solves the problem, only one
>>> thing is missing before I give my R-b: we need to clearly
>>> document what exactly are the consequences and requirements of
>>> setting kvm-hint-dedicated=on (I'm not sure if the best place for
>>> this is qemu-options.hx, x86_cpu_list(), or somewhere else).
>>
>> I don't think we have a good place for this kind of documentation,
>> unfortunately.  Right now it is mentioned in
>> Documentation/virtual/kvm/cpuid.txt.
> 
> With this patch, the QEMU option will do more than just setting
> the CPUID bit, that's why I miss more detailed documentation on
> the QEMU side.  But I agree we have no obvious place for that
> documentation.
> 
> In the worst case we can just add a code comment on top of
> feature_word_info[FEAT_KVM_HINTS].feat_names warning that
> kvm-hint-dedicated won't just enable the flag on CPUID and has
> other side-effects.

Maybe we should use "-realtime dedicated=on" instead of, or in addition
to kvm-hint-dedicated=on?

Thanks,

Paolo