linux-kernel - Re: [PATCH] KVM: s390: Enable specification exception interpretation

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Date:   Tue, 6 Jul 2021 17:16:17 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        "open list:KERNEL VIRTUAL MACHINE for s390 (KVM/s390)" 
        <kvm@...r.kernel.org>,
        "open list:S390" <linux-s390@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: s390: Enable specification exception interpretation

On 06.07.21 14:02, Christian Borntraeger wrote:
> 
> 
> 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@...ux.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@...ux.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 :-)

Well, okay then :)

So the plan for vSIE is to always keep it disabled? IIUC, one could 
similarly always forward the bit of set.


-- 
Thanks,

David / dhildenb