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Subject: Re: [PATCH] KVM: s390: Enable specification exception interpretation
To:     David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Janis Schoetterl-Glausch <scgl@linux.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>
Cc:     Claudio Imbrenda <imbrenda@linux.ibm.com>,
        "open list:KERNEL VIRTUAL MACHINE for s390 (KVM/s390)" 
        <kvm@vger.kernel.org>,
        "open list:S390" <linux-s390@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
References: <20210706114714.3936825-1-scgl@linux.ibm.com>
 <87k0m3hd7h.fsf@redhat.com> <194128c1-8886-5b8b-2249-5ec58b8e7adb@de.ibm.com>
 <be78ce5d-92e4-36bd-aa28-e32db0342a44@redhat.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <45690e80-5c7c-1e11-99d5-c0d1482755ad@de.ibm.com>
Date:   Tue, 6 Jul 2021 14:02:45 +0200
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On 06.07.21 13:59, David Hildenbrand wrote:
> On 06.07.21 13:56, Christian Borntraeger wrote:
>>
>>
>> On 06.07.21 13:52, Cornelia Huck wrote:
>>> On Tue, Jul 06 2021, Janis Schoetterl-Glausch <scgl@linux.ibm.com> wrote:
>>>
>>>> When this feature is enabled the hardware is free to interpret
>>>> specification exceptions generated by the guest, instead of causing
>>>> program interruption interceptions.
>>>>
>>>> This benefits (test) programs that generate a lot of specification
>>>> exceptions (roughly 4x increase in exceptions/sec).
>>>>
>>>> Interceptions will occur as before if ICTL_PINT is set,
>>>> i.e. if guest debug is enabled.
>>>>
>>>> Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
>>>> ---
>>>> I'll additionally send kvm-unit-tests for testing this feature.
>>>>
>>>>    arch/s390/include/asm/kvm_host.h | 1 +
>>>>    arch/s390/kvm/kvm-s390.c         | 2 ++
>>>>    arch/s390/kvm/vsie.c             | 2 ++
>>>>    3 files changed, 5 insertions(+)
>>>
>>> (...)
>>>
>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>> index b655a7d82bf0..aadd589a3755 100644
>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>> @@ -3200,6 +3200,8 @@ static int kvm_s390_vcpu_setup(struct kvm_vcpu *vcpu)
>>>>            vcpu->arch.sie_block->ecb |= ECB_SRSI;
>>>>        if (test_kvm_facility(vcpu->kvm, 73))
>>>>            vcpu->arch.sie_block->ecb |= ECB_TE;
>>>> +    if (!kvm_is_ucontrol(vcpu->kvm))
>>>> +        vcpu->arch.sie_block->ecb |= ECB_SPECI;
>>>
>>> Does this exist for any hardware version (i.e. not guarded by a cpu
>>> feature?)
>>
>> Not for all hardware versions, but also no indication. The architecture
>> says that the HW is free to do this or not. (which makes the vsie code
>> simpler).
> 
> I remember the architecture said at some point to never set undefined bits - and this bit is undefined on older HW generations. I might be wrong, though.

I can confirm that this bit will be ignored on older machines. The notion of
never setting undefined bits comes from "you never know what this bit will
change in future machines". Now we know :-)