Re: [PATCH v6 05/21] KVM: s390: CPU model support for AP virtualization

[Halil Pasic]

On 07/02/2018 06:11 PM, Cornelia Huck wrote:
> On Mon, 2 Jul 2018 11:54:28 -0400
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > On 07/02/2018 11:41 AM, Cornelia Huck wrote:
> > > On Mon, 2 Jul 2018 11:37:11 -0400
> > > Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
> > >   
> > > > On 07/02/2018 10:38 AM, Christian Borntraeger wrote:
> > > > > On 06/29/2018 11:11 PM, Tony Krowiak wrote:
> > > > > > Introduces a new CPU model feature and two CPU model
> > > > > > facilities to support AP virtualization for KVM guests.
> > > > > >
> > > > > > CPU model feature:
> > > > > >
> > > > > > The KVM_S390_VM_CPU_FEAT_AP feature indicates that
> > > > > > AP instructions are available on the guest. This
> > > > > > feature will be enabled by the kernel only if the AP
> > > > > > instructions are installed on the linux host. This feature
> > > > > > must be specifically turned on for the KVM guest from
> > > > > > userspace to use the VFIO AP device driver for guest
> > > > > > access to AP devices.
> > > > > >
> > > > > > CPU model facilities:
> > > > > >
> > > > > > 1. AP Query Configuration Information (QCI) facility is installed.
> > > > > >
> > > > > >       This is indicated by setting facilities bit 12 for
> > > > > >       the guest. The kernel will not enable this facility
> > > > > >       for the guest if it is not set on the host. This facility
> > > > > >       must not be set by userspace if the KVM_S390_VM_CPU_FEAT_AP
> > > > > >       feature is not installed.
> > > > > >
> > > > > >       If this facility is not set for the KVM guest, then only
> > > > > >       APQNs with an APQI less than 16 will be available to the
> > > > > >       guest regardless of the guest's matrix configuration. This
> > > > > >       is a limitation of the AP bus running on the guest.
> > > > > >
> > > > > > 2. AP Facilities Test facility (APFT) is installed.
> > > > > >
> > > > > >       This is indicated by setting facilities bit 15 for
> > > > > >       the guest. The kernel will not enable this facility for
> > > > > >       the guest if it is not set on the host. This facility
> > > > > >       must not be set by userspace if the KVM_S390_VM_CPU_FEAT_AP
> > > > > >       feature is not installed.
> > > > > >
> > > > > >       If this facility is not set for the KVM guest, then no
> > > > > >       AP devices will be available to the guest regardless of
> > > > > >       the guest's matrix configuration. This is a limitation
> > > > > >       of the AP bus running under the guest.
> > > > > >
> > > > > > Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> > > > > > Reviewed-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
> > > > > > Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>
> > > > > I think it probably should be at the end of the series, other than that its good.
> > > > If I move this to the end of the series, the very next patch checks the
> > > >
> > > > KVM_S390_VM_CPU_FEAT_AP feature?
> > > Introduce it here, offer it only with the last patch?
> >
> > I apologize, but I don't know what you mean by this. Are you suggesting
> > this patch
> > should only include the #define for KVM_S390_VM_CPU_FEAT_AP?
>
> Yes, just introduce the definition here (so code later in the series
> can refer to it) and flip the switch (offer the bit) as the final
> patch.

The other features introduced and exposed here are no different. For
KVM_S390_VM_CPU_FEAT_AP defer exposing means defer allow_cpu_feat();
for the STFLE features, defer adding to FACILITIES_KVM_CPUMODEL.

Anyway, I think the definition should be squashed into #6. Expose the
features after patch #6 is in place or expose them at the end of the
series is IMHO a matter of taste -- and I lean towards expose at the
end of the series.

Regards,
Halil