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

[Collin Walling]

On 6/26/19 8:14 AM, Cornelia Huck wrote:
> On Wed, 26 Jun 2019 11:12:04 +0200
> Christian Borntraeger <address@hidden> 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 :/

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.