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

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Date:   Tue, 27 Mar 2018 13:22:56 +0200
From:   Pierre Morel <pmorel@...ux.vnet.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>,
        Tony Krowiak <akrowiak@...ux.vnet.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        kwankhede@...dia.com, bjsdjshi@...ux.vnet.ibm.com,
        pbonzini@...hat.com, alex.williamson@...hat.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com
Subject: Re: [PATCH v3 03/14] KVM: s390: CPU model support for AP
 virtualization

On 27/03/2018 12:59, Cornelia Huck wrote:
> On Wed, 14 Mar 2018 14:25:43 -0400
> Tony Krowiak <akrowiak@...ux.vnet.ibm.com> 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@...ibm.com>
>> Reviewed-by: Halil Pasic <pasic@...ux.vnet.ibm.com>
>> Signed-off-by: Tony Krowiak <akrowiak@...ux.vnet.ibm.com>
>> ---
>>   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 Böblingen - Germany