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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

> -- 
> Eduardo