Re: [PATCH] KVM: s390: vsie: retry SIE instruction on host intercepts

[Janosch Frank <frankja@linux.ibm.com>]

On 4/29/24 12:18, Christian Borntraeger wrote:
> Am 04.03.24 um 16:37 schrieb Eric Farman:
>> On Mon, 2024-03-04 at 09:44 +0100, Christian Borntraeger wrote:
>>>
>>>
>>> Am 04.03.24 um 09:35 schrieb David Hildenbrand:
>>>> On 01.03.24 21:43, Eric Farman wrote:
>>>>> It's possible that SIE exits for work that the host needs to
>>>>> perform
>>>>> rather than something that is intended for the guest.
>>>>>
>>>>> A Linux guest will ignore this intercept code since there is
>>>>> nothing
>>>>> for it to do, but a more robust solution would rewind the PSW
>>>>> back to
>>>>> the SIE instruction. This will transparently resume the guest
>>>>> once
>>>>> the host completes its work, without the guest needing to process
>>>>> what is effectively a NOP and re-issue SIE itself.
>>>>
>>>> I recall that 0-intercepts are valid by the architecture. Further,
>>>> I recall that there were some rather tricky corner cases where
>>>> avoiding 0-intercepts would not be that easy.
>>
>> Any chance you recall any details of those corner cases? I can try to
>> chase some of them down.
>>
>>>>
>>>> Now, it's been a while ago, and maybe I misremember. SoI'm trusting
>>>> people with access to documentation can review this.
>>>
>>> Yes, 0-intercepts are allowed, and this also happens when LPAR has an
>>> exit.
>>
>>   From an offline conversation I'd had some months back:
>>
>> """
>> The arch does allow ICODE=0 to be stored, but it's supposed to happen
>> only upon a host interruption -- in which case the old PSW is supposed
>> to point back at the SIE, to resume guest execution if the host should
>> LPSW oldPSW.
>> """
> 
> Just re-read the architecture again and I agree, the SIE instruction should
> be nullified. So we should go forward with this somehow.
> 
> Eric, can you maybe add this to devel for CI coverage so that we see if there
> are corner cases? Maybe also try to do some performance things (how many IPIs
> can we get in guest2 when a guest3 is running and how many IPIs are possible
> in a guest3).
> 
> 

This patch has had contact with the CI for quite a while and I'm 
gathering patches.
Is this an ack now?