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

Emanuele Giuseppe Esposito
2022-09-18T16:13:45Z

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)."

Probably @Paolo and @Maxim can add more to this.

Thank you,
Emanuele