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

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Apr 19, 2018 at 11:32:16PM +0200, Paolo Bonzini wrote:
> 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?

Maybe it's a better idea than overloading an option that is only
expected to control a CPUID bit.

-- 
Eduardo