On Mon, 5 Aug 2019 10:54:40 +0200
Igor Mammedov <imamm...@redhat.com> wrote:
> On Fri, 2 Aug 2019 17:04:21 +0200
> Christian Borntraeger <borntrae...@de.ibm.com> wrote:
>
> > On 02.08.19 16:59, Christian Borntraeger wrote:
> > >
> > >
> > > On 02.08.19 16:42, Christian Borntraeger wrote:
> > >> On 02.08.19 15:32, Igor Mammedov wrote:
> > >>> Changelog:
> > >>> since v2:
> > >>> - break migration from old QEMU (since 2.12-4.1) for guest with
> > >>> >8TB RAM
> > >>> and drop migratable aliases patch as was agreed during v2 review
FWIW, that seems reasonable to me as well.
> > >>> - drop 4.2 machines patch as it's not prerequisite anymore
> > >>> since v1:
> > >>> - include 4.2 machines patch for adding compat RAM layout on top
> > >>> - 2/4 add missing in v1 patch for splitting too big MemorySection on
> > >>> several memslots
> > >>> - 3/4 amend code path on alias destruction to ensure that RAMBlock
> > >>> is
> > >>> cleaned properly
> > >>> - 4/4 add compat machine code to keep old layout (migration-wise)
> > >>> for
> > >>> 4.1 and older machines
> > >>>
> > >>>
> > >>> While looking into unifying guest RAM allocation to use hostmem backends
> > >>> for initial RAM (especially when -mempath is used) and retiring
> > >>> memory_region_allocate_system_memory() API, leaving only single hostmem
> > >>> backend,
> > >>> I was inspecting how currently it is used by boards and it turns out
> > >>> several
> > >>> boards abuse it by calling the function several times (despite
> > >>> documented contract
> > >>> forbiding it).
> > >>>
> > >>> s390 is one of such boards where KVM limitation on memslot size got
> > >>> propagated
> > >>> to board design and memory_region_allocate_system_memory() was abused
> > >>> to satisfy
> > >>> KVM requirement for max RAM chunk where memory region alias would
> > >>> suffice.
> > >>>
> > >>> Unfortunately, memory_region_allocate_system_memory() usage created
> > >>> migration
> > >>> dependency where guest RAM is transferred in migration stream as
> > >>> several RAMBlocks
> > >>> if it's more than KVM_SLOT_MAX_BYTES. During v2 review it was agreed to
> > >>> ignore
> > >>> migration breakage (documenting it in release notes) and leaving only
> > >>> KVM fix.
> > >>>
> > >>> In order to replace these several RAM chunks with a single memdev and
> > >>> keep it
> > >>> working with KVM memslot size limit, following was done:
> > >>> * [1/2] split too big RAM chunk inside of KVM code on several memory
> > >>> slots
> > >>> if necessary
> > >>> * [2/2] drop manual ram splitting in s390 code
> > >>>
> > >>>
> > >>> CC: pbonz...@redhat.com
> > >>> CC: qemu-s3...@nongnu.org
> > >>> CC: borntrae...@de.ibm.com
> > >>> CC: th...@redhat.com
> > >>> CC: da...@redhat.com
> > >>> CC: coh...@redhat.com
> > >>
> > >> With the fixup this patch set seems to work on s390. I can start 9TB
> > >> guests and
> > >> I can migrate smaller guests between 4.1+patch and 4.0 and 3.1. I
> > >> currently can
> > >> not test migration for the 9TB guest due to lack of a 2nd system.
> > >
> > > I have to correct myself. The 9TB guest started up but it does not seem
> > > to do
> > > anything useful (it hangs).
> >
> > Seems that the userspace addr is wrong (its the same).
> > [pid 258234] ioctl(10, KVM_SET_USER_MEMORY_REGION, {slot=0, flags=0,
> > guest_phys_addr=0, memory_size=8796091973632,
> > userspace_addr=0x3fff7d00000}) = 0
> > [pid 258234] ioctl(10, KVM_SET_USER_MEMORY_REGION, {slot=1, flags=0,
> > guest_phys_addr=0x7fffff00000, memory_size=1099512676352,
> > userspace_addr=0x3fff7d00000}) = 0
>
> It's a bug in 1/2, I forgot to advance mem->ram along with mem->start_addr.
> Let me fix it and simulate it on small s390 host (/me sorry for messy patches)
> it won't test migration properly but should be sufficient for testing KVM
> code patch.
>
Ok, I'll wait for a v4 before I spend any time on this :)
