Re: [Qemu-devel] [qemu-s390x] [PATCH for-4.2 v5 1/2] kvm: s390: split too big memory section on several memslots

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 30 Aug 2019 18:19:29 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> On 30.08.19 11:41, Igor Mammedov wrote:
> > On Thu, 29 Aug 2019 14:41:13 +0200
> > Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> >   
> >> On 29.08.19 14:31, Igor Mammedov wrote:  
> >>> On Thu, 29 Aug 2019 14:07:44 +0200
> >>> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> >>>     
> >>>> On 29.08.19 14:04, Igor Mammedov wrote:    
> >>>>> On Thu, 29 Aug 2019 08:47:49 +0200
> >>>>> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> >>>>>       
> >>>>>> On 27.08.19 14:56, Igor Mammedov wrote:      
> >>>>>>> On Tue, 20 Aug 2019 18:07:27 +0200
> >>>>>>> Cornelia Huck <cohuck@redhat.com> wrote:
> >>>>>>>         
> >>>>>>>> On Wed,  7 Aug 2019 11:32:41 -0400
> >>>>>>>> Igor Mammedov <imammedo@redhat.com> wrote:
> >>>>>>>>        
> >>>>>>>>> Max memslot size supported by kvm on s390 is 8Tb,
> >>>>>>>>> move logic of splitting RAM in chunks upto 8T to KVM code.
> >>>>>>>>>
> >>>>>>>>> This way it will hide KVM specific restrictions in KVM code
> >>>>>>>>> and won't affect baord level design decisions. Which would allow
> >>>>>>>>> us to avoid misusing memory_region_allocate_system_memory() API
> >>>>>>>>> and eventually use a single hostmem backend for guest RAM.
> >>>>>>>>>
> >>>>>>>>> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> >>>>>>>>> ---
> >>>>>>>>> v5:
> >>>>>>>>>   * move computation 'size -= slot_size' inside of loop body
> >>>>>>>>>           (David Hildenbrand <david@redhat.com>)
> >>>>>>>>> v4:
> >>>>>>>>>   * fix compilation issue
> >>>>>>>>>           (Christian Borntraeger <borntraeger@de.ibm.com>)
> >>>>>>>>>   * advance HVA along with GPA in kvm_set_phys_mem()
> >>>>>>>>>           (Christian Borntraeger <borntraeger@de.ibm.com>)
> >>>>>>>>>
> >>>>>>>>> patch prepares only KVM side for switching to single RAM memory region
> >>>>>>>>> another patch will take care of  dropping manual RAM partitioning in
> >>>>>>>>> s390 code.          
> >>>>>>>>
> >>>>>>>> I may have lost track a bit -- what is the status of this patch (and
> >>>>>>>> the series)?        
> >>>>>>>
> >>>>>>> Christian,
> >>>>>>>
> >>>>>>> could you test it on a host that have sufficient amount of RAM?        
> >>>>>>
> >>>>>>
> >>>>>> This version looks good. I was able to start a 9TB guest.
> >>>>>> [pid 215723] ioctl(10, KVM_SET_USER_MEMORY_REGION, {slot=0, flags=0, guest_phys_addr=0, memory_size=8796091973632, userspace_addr=0x3ffee700000}) = 0
> >>>>>> [pid 215723] ioctl(10, KVM_SET_USER_MEMORY_REGION, {slot=1, flags=0, guest_phys_addr=0x7fffff00000, memory_size=1099512676352, userspace_addr=0xbffee600000}) = 0    
> >>>>    
> >>>>>> The only question is if we want to fix the weird alignment (0x7fffff00000) when
> >>>>>> we already add a migration barrier for uber-large guests.
> >>>>>> Maybe we could split at 4TB to avoid future problem with larger page sizes?      
> >>>>> That probably should be a separate patch on top.      
> >>>>
> >>>> Right. The split in KVM code is transparent to migration and other parts of QEMU, correct?    
> >>>
> >>> it should not affect other QEMU parts and migration (to my limited understanding of it),
> >>> we are passing to KVM memory slots upto KVM_SLOT_MAX_BYTES as we were doing before by
> >>> creating several memory regions instead of one as described in [2/2] commit message.
> >>>
> >>> Also could you also test migration of +9Tb guest, to check that nothing where broken by
> >>> accident in QEMU migration code?    
> >>
> >> I only have one server that is large enough :-/  
> > Could you test offline migration on it (to a file and restore from it)?  
> 
> I tested migration with a hacked QEMU (basically split in KVM code at 1GB instead of 8TB) and
> the restore from file failed with data corruption in the guest. The current code
> does work when I use small memslots. No idea yet what is wrong.

I've tested 2Gb (max, I can test) guest (also hacked up version)
and it worked for me.
How do you test it and detect corruption so I could try to reproduce it locally?
(given it worked before, there is no much hope but I could try)