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

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Date:   Wed, 7 Jul 2021 10:56:53 +0200
From:   Janis Schoetterl-Glausch <scgl@...ux.vnet.ibm.com>
To:     Christian Borntraeger <borntraeger@...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 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.
> 
> 
> With that
> 
> Reviewed-by: Christian Borntraeger <borntraeger@...ibm.com>

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