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

[Eduardo Habkost <ehabkost@redhat.com>]

On Sat, May 12, 2018 at 01:12:59AM +0300, Michael S. Tsirkin wrote:
> On Thu, Apr 19, 2018 at 06:53:20PM -0300, Eduardo Habkost wrote:
> > 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.
> 
> Well -realtime would be a bit confusing in that it enables mlock by
> default.
> 
> From pure API point of view, hint-dedicated looks good since
> it seems to say "optimize for a dedicated host CPU" and
> it's a hint in that guests keep working if you violate this
> slightly once in a while.
> 
> But I agree there's a problem: right now "kvm-" means "KVM PV"
> as opposed to e.g. HV enlightened gusts.
> So if you enable hyperv and also want to disable halt existing,
> what then? What should kvm-hint-dedicated=on do?
> 
> So how about a new hint-dedicated=on cpu flag?  We can have that set
> kvm-hint-dedicated if kvm PV is enabled.

Using a boolean flag that is _not_ considered a CPUID feature
flag would be better.  Using the CPUID feature flag name risks
having management software enabling the flag by accident (as it
will get included in query-cpu-model-* queries).  A separate
boolean flag would make this clearer.

-- 
Eduardo