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

[Igor Mammedov]

On Thu, 29 Aug 2019 14:07:44 +0200
Christian Borntraeger <address@hidden> wrote:

> On 29.08.19 14:04, Igor Mammedov wrote:
> > On Thu, 29 Aug 2019 08:47:49 +0200
> > Christian Borntraeger <address@hidden> wrote:
> >   
> >> On 27.08.19 14:56, Igor Mammedov wrote:  
> >>> On Tue, 20 Aug 2019 18:07:27 +0200
> >>> Cornelia Huck <address@hidden> wrote:
> >>>     
> >>>> On Wed,  7 Aug 2019 11:32:41 -0400
> >>>> Igor Mammedov <address@hidden> 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 <address@hidden>
> >>>>> ---
> >>>>> v5:
> >>>>>   * move computation 'size -= slot_size' inside of loop body
> >>>>>           (David Hildenbrand <address@hidden>)
> >>>>> v4:
> >>>>>   * fix compilation issue
> >>>>>           (Christian Borntraeger <address@hidden>)
> >>>>>   * advance HVA along with GPA in kvm_set_phys_mem()
> >>>>>           (Christian Borntraeger <address@hidden>)
> >>>>>
> >>>>> 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?