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Re: [PATCH v3] s390/kvm: fix diag318 propagation and reset functionality

From: Christian Borntraeger
Subject: Re: [PATCH v3] s390/kvm: fix diag318 propagation and reset functionality
Date: Tue, 24 Nov 2020 19:14:43 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1

On 24.11.20 18:58, Collin Walling wrote:
> On 11/24/20 11:51 AM, Thomas Huth wrote:
>> On 24/11/2020 17.26, Collin Walling wrote:
>>> On 11/24/20 8:04 AM, Christian Borntraeger wrote:
>>>> On 24.11.20 13:48, Christian Borntraeger wrote:
>>>>> Now that we have this - fully implemented in QEMU - shouldnt
>>>>> we add els and diag318 also to the default CPU models?
>>>> At least the extended SCCB should be fine to be enabled all the 
>>>> time.
>>> Yeah ELS should be fine to default-enable since it's handled entirely
>>> via QEMU. If the kernel can support it, great. If not, then the kernel
>>> will simply provide the original 4K SCCB.
>>> DIAG318 requires KVM support, so I do not think we should default-enable
>>> that one :)
>> Ok, but does ELS have any advantages as long as you don't use diag318? If we
>> do not enable diag318 by default, what sense does it make to enable ELS by
>> default?
>>  Thomas
> One immediate benefit is that it would make configuring a guest to
> enable diag318 require one less step. Right now, the VM must explicitly
> enable both ELS *and* diag318 in order to use the latter feature. With
> ELS default-enabled, the user would only have to explicitly enable
> diag318 if they want it.

And it would allow to go beyond 248 CPUs. But since we are already late in 5.2, 
we can defer this to the next QEMU I guess.

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