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Subject: Re: [PATCH] KVM: s390: Enable specification exception interpretation
To:     Janis Schoetterl-Glausch <scgl@linux.vnet.ibm.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:     David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        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>
 <05430c91-6a84-0fc9-0af4-89f408eb691f@de.ibm.com>
 <c61223e4-0076-18c1-64bd-8ba899e91eb4@linux.vnet.ibm.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <a2e3484a-34af-0bab-2fb3-b22a69361807@de.ibm.com>
Date:   Wed, 7 Jul 2021 10:59:29 +0200
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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?