Re: [Qemu-devel] [qemu-s390x] [PATCH v5 2/2] s390: diagnose 318 info reset and migration support

[David Hildenbrand <david@redhat.com>]

On 26.06.19 16:22, Collin Walling wrote:
> On 6/26/19 8:14 AM, Cornelia Huck wrote:
>> On Wed, 26 Jun 2019 11:12:04 +0200
>> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
>>
>>> On 25.06.19 17:17, Collin Walling wrote:
>>>> index a606547..4c26754 100644
>>>> --- a/target/s390x/cpu.h
>>>> +++ b/target/s390x/cpu.h
>>>> @@ -39,7 +39,13 @@
>>>>   
>>>>   #define MMU_USER_IDX 0
>>>>   
>>>> -#define S390_MAX_CPUS 248
>>>> +/*
>>>> + * HACK: The introduction of additional facility bytes in the Read Info
>>>> + * struct consumes space used for CPU entries, thus we must reduce the
>>>> + * original maximum CPUs of 248 by one for each new byte or risk smashing
>>>> + * the stack.
>>>> + */
>>>> +#define S390_MAX_CPUS 247
>>>
>>> I think we decided to not change that. Only if the cpu model contains the diag318
>>> feature we are limited to 247 but only for the sclp response.
>>> So we said:
>>> - we continue to allow 248 cpus
>>> - the sclp response will be limited to 247 CPUs if the feature is one
>>> - (optional) we print a warning that the guest might not see all CPUs
>>>
>>
>> Yes, that's what I remember as well... and printing/logging a warning
>> is a good idea.
>>
> 
> I recall this conversation, but I encountered a bit of a hangup when
> running some tests with the new changes.
> 
> Since we're adding a new field in the ReadInfo struct, we're permanently
> intruding on the space used for CPU entries. A machine with these 
> changes and 248 CPUs resulted in stack smash when the guest would start 
> up. This happened with diag318 on *and* off. This is a limitation to the
> 4k SCCB size right now :/

I think we discussed to only indicate 247 CPUs, which should solve the
issue?

> 
> Prior to these patches, I was restricting the max_cpus depending on the
> compat version. I failed to do some tests with earlier versions to catch
> this error (there are a lot of moving parts... sorry).
> 
> I do not think the new byte and the full 248 CPU count can co-exist. We
> might be able to union byte 134 and some extra space with the first CPU 
> entry... but that could get messy.
> 
> 


-- 

Thanks,

David / dhildenb