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

From: Pierre Morel
Date: Thu Aug 23 2018 - 07:10:18 EST

On 23/08/2018 12:28, David Hildenbrand wrote:
On 23.08.2018 12:00, Halil Pasic wrote:


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?


Me too :) , I was assuming this block to be 128bit, but the qci block
has 128 bytes....

And looking at arch/s390/include/asm/ap.h, there is a lot of information
contained that is definitely not of interest for CPU models...

I wonder if there is somewhere defined which bits are reserved for
future features/facilities, compared to ap masks and such.

This is really hard to understand/plan without access to documentation.

You (Halil, Tony, Pier, ...) should have a look if what I described
related to PQAP(QCI) containing features that should get part of the CPU
model makes sense or not. For now I was thinking that there is some part
inside of QCI that is strictly reserved for facilities/features that we
can use.


David,
I already answered to you on this subject.

First,
Are you sure you do not mistake QCI for TAPQ which has the t bit
instruction interception bit as all the instructions you use as
subfunctions?

Second,
The TAPQ interception bit is exposed through the facility bit 15
and is documented as being installed when the APXA facility is installed.

If we have the APFT, we have the APXA, problem seems solved to me.

Regards,
Pierre







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