Re: [Qemu-devel] [qemu-s390x] [PATCH 1/1] s390x/sclp: fix maxram calculation

[Cornelia Huck]

On Mon, 30 Jul 2018 17:20:25 +0200
Christian Borntraeger <address@hidden> wrote:

> On 07/30/2018 05:17 PM, David Hildenbrand wrote:
> > On 30.07.2018 17:00, Christian Borntraeger wrote:  
> >>
> >>
> >> On 07/30/2018 04:34 PM, David Hildenbrand wrote:  
> >>> On 30.07.2018 16:09, Christian Borntraeger wrote:  
> >>>> We clamp down ram_size to match the sclp increment size. We do
> >>>> not do the same for maxram_size, which means for large guests
> >>>> with some sizes (e.g. -m 50000) maxram_size differs from ram_size.
> >>>> This can break other code (e.g. CMMA migration) which uses maxram_size
> >>>> to calculate the number of pages and then throws some errors.  
> >>>
> >>> So the only problem is that the buffer size between source and target
> >>> differ?  
> >>
> >> The problem is that the target tries to access a non-existing buffer when 
> >> committing all cmma value, so the kernel returns with EFAULT.  
> >>>  
> > 
> > Am I wrong or does CMMA migration code really not care about which parts
> > of maxram are actually used (== which memory regions are actually defined)?
> > 
> > If so, this looks broken to me and the right fix is to use ramsize for
> > now, because it simply does not support maxram.
> > 
> > (I assume using some -m X,maxmem=X+Y would make it fail in the same way)
> > 
> > (this patch still makes sense and should be done)  
> 
> I am looking for the minimal fix for 2.13 and ideally even for 2.12.1.

2.13 is not the QEMU version you are looking for :)

> Can we agree on this fix and do the remaining thing later?

If you guys agree, I'm happy to queue it to s390-fixes for 3.0-rc3.
Just be quick, as I'd need to send it tomorrow morning the latest :)

(FWIW, patch looks sane and small enough to me.)