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

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Date:   Wed, 7 Jul 2021 10:59:29 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.vnet.ibm.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:     David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        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 07.07.21 10:56, Janis Schoetterl-Glausch wrote:
> On 7/7/21 10:30 AM, Christian Borntraeger wrote:
>>
>>
>> On 06.07.21 13:47, Janis Schoetterl-Glausch 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.
>>
>> I think I will add
>>
>> There is no indication if this feature is available or not and the hardware
>> is free to interpret or not. So we can simply set this bit and if the
>> hardware ignores it we fall back to intercept 8 handling.
> 
> Might also mention vSIE and/or incorporate into first paragraph:
> 
> When this feature is enabled the hardware is free to interpret
> specification exceptions generated by the guest, instead of causing
> program interruption interceptions, but it is not required to.
> There is no indication if this feature is available or not,
> so we can simply set this bit and if the hardware ignores it
> we fall back to intercept 8 handling.
> The same applies to vSIE, we forward the guest hypervisor's bit
> and fall back to injection if interpretation does not occur.

Can you maybe resend a v2 with all comments (and RBs) added?