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

[David Hildenbrand <david@redhat.com>]

On 07.11.21 09:14, Michael S. Tsirkin wrote:
> On Tue, Nov 02, 2021 at 06:10:13PM +0100, David Hildenbrand wrote:
>> 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.
> 
> So, if the focus is performance improvement, maybe show the speedup?

Let's not focus on b), a) is the primary goal of this series:

"
a) Reduce the metadata overhead, including bitmap sizes inside KVM but
also inside QEMU KVM code where possible.
"

Because:

"
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.
"

Partially tackling b) is just a nice side effect of this series. In the
long term, we'll want userfaultfd-based protection, and I'll do a
performance evaluation then, how userfaultf vs. !userfaultfd compares
(boot time, run time, THP consumption).

I'll adjust the cover letter for the next version to make this clearer.

-- 
Thanks,

David / dhildenb