Re: [PATCH v3 03/14] KVM: s390: CPU model support for AP virtualization

[Pierre Morel]

On 27/03/2018 12:59, Cornelia Huck wrote:
> On Wed, 14 Mar 2018 14:25:43 -0400
> Tony Krowiak <akrowiak@xxxxxxxxxxxxxxxxxx> 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.
> >
> > 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.
> >
> > Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> > Reviewed-by: Halil Pasic <pasic@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxxxxxxx>
> > ---
> >   arch/s390/include/asm/kvm_host.h |    1 +
> >   arch/s390/include/uapi/asm/kvm.h |    1 +
> >   arch/s390/kvm/kvm-s390.c         |    4 ++++
> >   arch/s390/tools/gen_facilities.c |    2 ++
> >   4 files changed, 8 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > index c47731d..a60c45b 100644
> > --- a/arch/s390/kvm/kvm-s390.c
> > +++ b/arch/s390/kvm/kvm-s390.c
> > @@ -350,6 +350,10 @@ static void kvm_s390_cpu_feat_init(void)
> >   
> >   	if (MACHINE_HAS_ESOP)
> >   		allow_cpu_feat(KVM_S390_VM_CPU_FEAT_ESOP);
> > +
> > +	if (ap_instructions_installed()) /* AP instructions installed on host */
> > +		allow_cpu_feat(KVM_S390_VM_CPU_FEAT_AP);
> That's another dependency of the base kvm-s390 module on zcrypt, which
> I don't like at all.

In fact there is a tricky thing about zcrypt it is that even it is 
configured a a module
CONFIG_ZCRYPT=M, the AP BUS is built statically with the kernel.
See drivers/s390/crypto/Makefile
"
ap-objs := ap_bus.o ap_card.o ap_queue.o
obj-$(subst m,y,$(CONFIG_ZCRYPT)) += ap.o
"
ugly isn't it?

> There are two possibilities here:
> - Exposing the features makes sense even if no zcrypt driver is active
>    in the host. Then, ap_instructions_installed() needs to be moved into
>    always-built code (see my comments for the interface in patch 1).

This is what we need for future enhancement I think.

> - Exposing the features makes sense only if we actually want to make
>    vfio-ap available. Then we should provide the proper check in the
>    vfio-ap parts (which depends on zcrypt) and stub it out if vfio-ap is
>    not configured.
>
> > +
> >   	/*
> >   	 * We need SIE support, ESOP (PROT_READ protection for gmap_shadow),
> >   	 * 64bit SCAO (SCA passthrough) and IDTE (for gmap_shadow unshadowing).


--
Pierre Morel
Linux/KVM/QEMU in BÃblingen - Germany