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

David Hildenbrand
2022-09-19T07:53:48Z

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.

--
Thanks,

David / dhildenb