[PATCH v1 02/10] qemu_domain.c: align memory modules before calculating 'initialmem'

[Daniel Henrique Barboza]

On 11/13/20 10:51 AM, Andrea Bolognani wrote:
> On Fri, 2020-11-13 at 09:58 -0300, Daniel Henrique Barboza wrote:
>> On 11/13/20 7:30 AM, Andrea Bolognani wrote:
>>> On Wed, 2020-11-11 at 19:07 -0300, Daniel Henrique Barboza wrote:
>>>> +++ b/tests/qemuxml2argvdata/memory-hotplug-ppc64-nonuma.args
>>>> @@ -11,7 +11,7 @@ QEMU_AUDIO_DRV=none \
>>>>    -name QEMUGuest1 \
>>>>    -S \
>>>>    -machine pseries,accel=kvm,usb=off,dump-guest-core=off \
>>>> --m size=1310720k,slots=16,maxmem=4194304k \
>>>> +-m size=1048576k,slots=16,maxmem=4194304k \
>>>>    -realtime mlock=off \
>>>>    -smp 1,sockets=1,cores=1,threads=1 \
>>>>    -object memory-backend-ram,id=memdimm0,size=536870912 \
>>>
>>> This doesn't look right: AFAIK the initial memory size is
>>> guest-visible, so by changing how the alignment is performed you
>>> might both change the guest ABI across guest boots (if libvirt is
>>> upgraded in between them) and break migration (if either the source
>>> or destination host is running the newer libvirt but the other side
>>> isn't).
>>
>> Good point. I failed to consider ABI stability for ppc64 guest migration.
>> Yes, this will break older guests that happen to have extra memory. In
>> fact, this can be specially harmful for migration.
>>
>> This means that I can't proceed with any other ppc64 changes made here.
>> Aligning ppc64 DIMMs in PostParse will achieve the same results even
>> without this patch - the DIMMs will be aligned before qemuDomainAlignMemorySizes()
>> and initialmem will be rounded to a more precise value of 'currentMemory'.
>> I can think of ways to align ppc64 DIMMs while not touching initialmem,
>> but all of them will require extra state in the domain definition. The
>> benefits are there (the DIMMs will be aligned in the live XML) but I'm
>> not sure it's worth the extra code.
> 
> I only skimmed the remaining patches and didn't dig into the code as
> much, or as recently, as you have, but from a high-level perspective
> I don't see why you wouldn't be able to simply move the existing
> rounding logic from the command line generator to PostParse? It's not
> like the former has access to additional information that the latter
> can't get to, right?

Interesting. I suppose that moving the logic of "qemuDomainAlignMemorySizes"
to PostParse might allows us to align DIMMs while being able to not touch
'initialmem'. I'm not entirely sure if that code is dependent on runtime stuff
though (the NUMA related code for instance). I'll investigate.


DHB


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