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

[David Hildenbrand]

On 27.03.19 18:08, Igor Mammedov wrote:
> On Wed, 27 Mar 2019 15:01:37 +0100
> David Hildenbrand <address@hidden> wrote:
> 
>> On 27.03.19 10:09, Igor Mammedov wrote:
>>> On Wed, 27 Mar 2019 09:10:01 +0100
>>> David Hildenbrand <address@hidden> wrote:
>>>
>>>> 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]);
>>> Looking more at it, it is seems s390 is 'broken' anyways.
>>> We call qemu_getrampagesize() here with huge page backends on CLI
>>> but memory-backends are initialized later
>>>  qemu_opts_foreach(..., object_create_delayed, ...)
>>> so s390 doesn't take into account memory backends currently
>>
>> BTW that might indeed be true, we only check against --mem-path.
> 
> It's possible to break it with '-numa node,memdev=...' since we don't really 
> have
> anything to block that call chain for s390 (but I'd argue it's invalid use of 
> CLI
> for s390 but it's effectively -mem-path on steroids alternative)
> 

I remember that -numa on s390x is completely blocked, but my mind might
play tricks with me. Anyhow, detecting the biggest page size also ahs to
be fixed.

-- 

Thanks,

David / dhildenb