Re: [PATCH v1 00/12] virtio-mem: Expose device memory via multiple memslots

[David Hildenbrand <david@redhat.com>]

On 02.11.21 18:06, Michael S. Tsirkin wrote:
> On Tue, Nov 02, 2021 at 12:55:17PM +0100, David Hildenbrand wrote:
>> On 02.11.21 12:35, Michael S. Tsirkin wrote:
>>> On Tue, Nov 02, 2021 at 09:33:55AM +0100, David Hildenbrand wrote:
>>>> On 01.11.21 23:15, Michael S. Tsirkin wrote:
>>>>> On Wed, Oct 27, 2021 at 02:45:19PM +0200, David Hildenbrand wrote:
>>>>>> This is the follow-up of [1], dropping auto-detection and vhost-user
>>>>>> changes from the initial RFC.
>>>>>>
>>>>>> Based-on: 20211011175346.15499-1-david@redhat.com
>>>>>>
>>>>>> A virtio-mem device is represented by a single large RAM memory region
>>>>>> backed by a single large mmap.
>>>>>>
>>>>>> Right now, we map that complete memory region into guest physical addres
>>>>>> space, resulting in a very large memory mapping, KVM memory slot, ...
>>>>>> although only a small amount of memory might actually be exposed to the VM.
>>>>>>
>>>>>> For example, when starting a VM with a 1 TiB virtio-mem device that only
>>>>>> exposes little device memory (e.g., 1 GiB) towards the VM initialliy,
>>>>>> in order to hotplug more memory later, we waste a lot of memory on metadata
>>>>>> for KVM memory slots (> 2 GiB!) and accompanied bitmaps. Although some
>>>>>> optimizations in KVM are being worked on to reduce this metadata overhead
>>>>>> on x86-64 in some cases, it remains a problem with nested VMs and there are
>>>>>> other reasons why we would want to reduce the total memory slot to a
>>>>>> reasonable minimum.
>>>>>>
>>>>>> We want to:
>>>>>> a) Reduce the metadata overhead, including bitmap sizes inside KVM but also
>>>>>>    inside QEMU KVM code where possible.
>>>>>> b) Not always expose all device-memory to the VM, to reduce the attack
>>>>>>    surface of malicious VMs without using userfaultfd.
>>>>>
>>>>> I'm confused by the mention of these security considerations,
>>>>> and I expect users will be just as confused.
>>>>
>>>> Malicious VMs wanting to consume more memory than desired is only
>>>> relevant when running untrusted VMs in some environments, and it can be
>>>> caught differently, for example, by carefully monitoring and limiting
>>>> the maximum memory consumption of a VM. We have the same issue already
>>>> when using virtio-balloon to logically unplug memory. For me, it's a
>>>> secondary concern ( optimizing a is much more important ).
>>>>
>>>> Some users showed interest in having QEMU disallow access to unplugged
>>>> memory, because coming up with a maximum memory consumption for a VM is
>>>> hard. This is one step into that direction without having to run with
>>>> uffd enabled all of the time.
>>>
>>> Sorry about missing the memo - is there a lot of overhead associated
>>> with uffd then?
>>
>> When used with huge/gigantic pages, we don't particularly care.
>>
>> For other memory backends, we'll have to route any population via the
>> uffd handler: guest accesses a 4k page -> place a 4k page from user
>> space. Instead of the kernel automatically placing a THP, we'd be
>> placing single 4k pages and have to hope the kernel will collapse them
>> into a THP later.
> 
> How much value there is in a THP given it's not present?

If you don't place a THP right during the first page fault inside the
THP region, you'll have to rely on khugepagd to eventually place a huge
page later -- and manually fault in each and every 4k page. I haven't
done any performance measurements so far. Going via userspace on every
4k fault will most certainly hurt performance when first touching memory.

-- 
Thanks,

David / dhildenb