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 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)?