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

[Eduardo Habkost]

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.

-- 
Eduardo