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

[Michael S. Tsirkin]

On Wed, May 16, 2018 at 02:44:24PM +0200, Paolo Bonzini wrote:
> On 12/05/2018 00:12, Michael S. Tsirkin wrote:
> >> 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.
> 
> Currently, the only suboption of "-realtime" is mlock, which means that
> the only three valid uses of it are
> 
>       -realtime mlock=on
>       -realtime mlock=off
>       -realtime ''
> 
> We can change the default, I think.  Only the third would change
> meaning, and it's a slightly crazy way to use the option.
> 
> > 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?
> 
> kvm-hint-dedicated=on only sets the CPUID bit, which Linux for example
> uses that to disable pv spinlocks.  "-realtime dedicated-cpus=on" only
> disables the vmexits.  You can use the two independently.

But when would you want to use the two independently?

> Thanks,
> 
> Paolo
> 
> > So how about a new hint-dedicated=on cpu flag?  We can have that set
> > kvm-hint-dedicated if kvm PV is enabled.
> >