Re: [qemu-s390x] [Qemu-devel] [PATCH for-4.0?] exec: Only count mapped memory backends for qemu_getrampagesize()

[David Hildenbrand]

On 27.03.19 01:12, David Gibson wrote:
> On Tue, Mar 26, 2019 at 06:02:51PM +0100, David Hildenbrand wrote:
>> On 26.03.19 15:08, Igor Mammedov wrote:
>>> On Tue, 26 Mar 2019 14:50:58 +1100
>>> David Gibson <address@hidden> wrote:
>>>
>>>> qemu_getrampagesize() works out the minimum host page size backing any of
>>>> guest RAM.  This is required in a few places, such as for POWER8 PAPR KVM
>>>> guests, because limitations of the hardware virtualization mean the guest
>>>> can't use pagesizes larger than the host pages backing its memory.
>>>>
>>>> However, it currently checks against *every* memory backend, whether or not
>>>> it is actually mapped into guest memory at the moment.  This is incorrect.
>>>>
>>>> This can cause a problem attempting to add memory to a POWER8 pseries KVM
>>>> guest which is configured to allow hugepages in the guest (e.g.
>>>> -machine cap-hpt-max-page-size=16m).  If you attempt to add non-hugepage,
>>>> you can (correctly) create a memory backend, however it (correctly) will
>>>> throw an error when you attempt to map that memory into the guest by
>>>> 'device_add'ing a pc-dimm.
>>>>
>>>> What's not correct is that if you then reset the guest a startup check
>>>> against qemu_getrampagesize() will cause a fatal error because of the new
>>>> memory object, even though it's not mapped into the guest.
>>> I'd say that backend should be remove by mgmt app since device_add failed
>>> instead of leaving it to hang around. (but fatal error either not a nice
>>> behavior on QEMU part)
>>
>> Indeed, it should be removed. Depending on the options (huge pages with
>> prealloc?) memory might be consumed for unused memory. Undesired.
> 
> Right, but if the guest initiates a reboot before the management gets
> to that, we'll have a crash.
> 

Yes, I agree.

At least on s390x (extending on what Igor said):

mc->init() -> s390_memory_init() ->
memory_region_allocate_system_memory() -> host_memory_backend_set_mapped()


ac->init_machine() -> kvm_arch_init() ->
kvm_s390_configure_mempath_backing() -> qemu_getrampagesize()


And in vl.c

configure_accelerator(current_machine, argv[0]);
...
machine_run_board_init()

So memory is indeed not mapped before calling qemu_getrampagesize().


We *could* move the call to kvm_s390_configure_mempath_backing() to
s390_memory_init().

cap_hpage_1m is not needed before we create VCPUs, so this would work fine.

We could than eventually make qemu_getrampagesize() asssert if no
backends are mapped at all, to catch other user that rely on this being
correct.

-- 

Thanks,

David / dhildenb