Re: [PATCH v9 21/22] KVM: s390: CPU model support for AP virtualization

[Tony Krowiak]

On 08/22/2018 07:19 AM, David Hildenbrand wrote:
> On 13.08.2018 23:48, Tony Krowiak wrote:
> > From: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>
> >
> > 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.
> >
> >     If this facility is not set for the KVM guest, then only
> >     APQNs with an APQI less than 16 will be used by a Linux
> >     guest regardless of the matrix configuration for the virtual
> >     machine. This is a limitation of the Linux AP bus.
> >
> > 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.
> >
> >     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 for the virtual
> >     machine. This is a limitation of the Linux AP bus.
> >
> > Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>
> > Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> > Reviewed-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
> > Tested-by: Michael Mueller <mimu@xxxxxxxxxxxxx>
> > Tested-by: Farhan Ali <alifm@xxxxxxxxxxxxx>
> > Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> > ---
> >   arch/s390/kvm/kvm-s390.c         |    5 +++++
> >   arch/s390/tools/gen_facilities.c |    2 ++
> >   2 files changed, 7 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > index 1e8cb67..d5e04d2 100644
> > --- a/arch/s390/kvm/kvm-s390.c
> > +++ b/arch/s390/kvm/kvm-s390.c
> > @@ -367,6 +367,11 @@ static void kvm_s390_cpu_feat_init(void)
> >   
> >   	if (MACHINE_HAS_ESOP)
> >   		allow_cpu_feat(KVM_S390_VM_CPU_FEAT_ESOP);
> > +
> > +	/* Check if AP instructions installed on host */
> > +	if (ap_instructions_available())
> > +		allow_cpu_feat(KVM_S390_VM_CPU_FEAT_AP);
> > +
> >   	/*
> >   	 * We need SIE support, ESOP (PROT_READ protection for gmap_shadow),
> >   	 * 64bit SCAO (SCA passthrough) and IDTE (for gmap_shadow unshadowing).
> > diff --git a/arch/s390/tools/gen_facilities.c b/arch/s390/tools/gen_facilities.c
> > index 90a8c9e..a52290b 100644
> > --- a/arch/s390/tools/gen_facilities.c
> > +++ b/arch/s390/tools/gen_facilities.c
> > @@ -106,6 +106,8 @@ struct facility_def {
> >   
> >   		.name = "FACILITIES_KVM_CPUMODEL",
> >   		.bits = (int[]){
> > +			12, /* AP Query Configuration Information */
> > +			15, /* AP Facilities Test */
> >   			-1  /* END */
> >   		}
> >   	},
> I really wonder if we should also export the APXA facility.

Given this comment is made within the context of the
FACILITIES_KVM_CPUMODEL I might point out that APXA is not
indicated by a facilities bit. It is indicated by a bit in
the QCI control block returned from the PQAP(QCI)
instruction to indicate that APXA is installed on all CPUs.

> We can probe and allow that CPU feature. However, we cannot disable it
> (as of now).

Given this patch series implements passthrough devices,
the output of the PQAP(QCI) will always be from a real
device - i.e., there will be no way to disable it.

> We have other CPU features where it is the same case (basically all
> subfunctions). See kvm_s390_get_processor_subfunc(). We probe them and
> export them, but support to disable them has never been implemented.
>
> On a high level, we could then e.g. deny to start a QEMU guest if APXA
> is available but has been disabled. (until we know that disabling it
> actually works - if ever).
>
> This helps to catch nasty migration bugs (e.g. APXA suddenly
> disappearing). Although unlikely, definitely possible.

Migration of AP devices is not supported by this patch series, so this 
should
not be an issue.

> Are there any other AP related facilities that the guest can from now on
> probe that should also become part of the CPU model?