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

[Halil Pasic]

On 08/23/2018 09:44 AM, David Hildenbrand wrote:
> On 22.08.2018 22:16, Tony Krowiak wrote:
> > On 08/22/2018 07:24 AM, David Hildenbrand wrote:
> > > On 22.08.2018 13:19, 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.
> > > >
> > > > We can probe and allow that CPU feature. However, we cannot disable it
> > > > (as of now).
> > > >
> > > > 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.
> > > >
> > > >
> > > > Are there any other AP related facilities that the guest can from now on
> > > > probe that should also become part of the CPU model?
> > > To be more precise, shouldn't PQAP(QCI) be handled just like other
> > > subfunctions? (I remember it should)
> >
> > When you suggest PQAP(QCI) be handled like other subfunctions, are you
> > suggesting that there should be a field in struct kvm_s390_vm_cpu_subfunc
> > with a bit indicating the QCI subfunction is available? The availability
> > of the QCI subfunction of the PQAP instruction is determined by facilities
> > bit 12. Is it not enough to export facilities bit 12?
>
> The feature block (128 bit) from PQAP(QCI) should be passed through a
> subfunction block to QEMU.

I'm confused, which 128 bit?

> So it is about passing e.g. APXA availability, not QCI itself. (as you
> correctly said, that is stfl 12)