Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates

David Hildenbrand
2022-09-19T17:30:44Z

On 19.09.22 09:53, David Hildenbrand wrote:
> On 18.09.22 18:13, Emanuele Giuseppe Esposito wrote:
>>
>>
>> Am 09/09/2022 um 16:30 schrieb Sean Christopherson:
>>> On Fri, Sep 09, 2022, Emanuele Giuseppe Esposito wrote:
>>>> KVM is currently capable of receiving a single memslot update through
>>>> the KVM_SET_USER_MEMORY_REGION ioctl.
>>>> The problem arises when we want to atomically perform multiple updates,
>>>> so that readers of memslot active list avoid seeing incomplete states.
>>>>
>>>> For example, in RHBZ https://bugzilla.redhat.com/show_bug.cgi?id=1979276
>>>
>>> I don't have access. Can you provide a TL;DR?
>>
>> You should be able to have access to it now.
>>
>>>
>>>> we see how non atomic updates cause boot failure, because vcpus
>>>> will se a partial update (old memslot delete, new one not yet created)
>>>> and will crash.
>>>
>>> Why not simply pause vCPUs in this scenario? This is an awful lot of a complexity
>>> to take on for something that appears to be solvable in userspace.
>>>
>>
>> I think it is not that easy to solve in userspace: see
>> https://lore.kernel.org/qemu-devel/20200312161217.3590-1-david@redhat.com/
>>
>>
>> "Using pause_all_vcpus()/resume_all_vcpus() is not possible, as it will
>> temporarily drop the BQL - something most callers can't handle (esp.
>> when called from vcpu context e.g., in virtio code)."
>
> Can you please comment on the bigger picture? The patch from me works
> around *exactly that*, and for that reason, contains that comment.
>

FWIW, I hacked up my RFC to perform atomic updates on any memslot
transactions (not just resizes) where ranges do add overlap with ranges
to remove.

https://github.com/davidhildenbrand/qemu/tree/memslot

I only performed simple boot check under x86-64 (where I can see region
resizes) and some make checks -- pretty sure it has some rough edges;
but should indicate what's possible and what the possible price might
be. [one could wire up a new KVM ioctl and call it conditionally on
support if really required]

--
Thanks,

David / dhildenb