Re: [Qemu-devel] [PATCH v3 0/3] pc-dimm: factor out MemoryDevice

[David Hildenbrand]

On 23.04.2018 17:32, Pankaj Gupta wrote:
> 
> Hi Igor,
> 
>>
>>> Right now we can only map PCDIMM/NVDIMM into guest address space. In the
>>> future, we might want to do the same for virtio devices - e.g.
>>> virtio-pmem or virtio-mem. Especially, they should be able to live side
>>> by side to each other.
>>>
>>> E.g. the virto based memory devices regions will not be exposed via ACPI
>>> and friends. They will be detected just like other virtio devices and
>>> indicate the applicable memory region. This makes it possible to also use
>>> them on architectures without memory device detection support (e.g. s390x).
>>>
>>> Let's factor out the memory device code into a MemoryDevice interface.
>> A couple of high level questions as relevant code is not here:
>>
>>   1. what would hotplug/unplug call chain look like in case of virtio-pmem
>>   device
>>      (reason I'm asking is that pmem being PCI device would trigger
>>       PCI bus hotplug controller and then it somehow should piggyback
>>       to Machine provided hotplug handlers, so I wonder what kind of
>>       havoc it would cause on hotplug infrastructure)
> 
> For first phase we are using 'virtio-pmem' as cold added devices. AFAIU
> 'VirtioDeviceClass' being parent class and 'hotplug/unplug' methods 
> implemented 
> for virtio-pmem device. So, pci bus hotplug/unplug should call the 
> corresponding
> functions?
> 
>>
>>   2. why not use PCI bar mapping mechanism to do mapping since pmem is PCI
>>   device?
> 
> I think even we use if as PCI BAR mapping with PCI we still need free guest 
> physical 
> address to provide to VM for mapping the memory range. For that there needs 
> to 
> be coordination between PCDIMM and VIRTIO pci device? Also, if we use RAM 
> from QEMU 
> address space tied to big region(system_memory) memory accounting gets easier 
> and at single place.
> 
> Honestly speaking I don't think there will be much difference between the two 
> approaches? unless
> I am missing something important?

The difference by gluing virtio devices to architecture specific
technologies will be unnecessary complicated. (my humble opinion)

-- 

Thanks,

David / dhildenb