Messages in this thread | | | Date | Sun, 18 Sep 2022 18:13:31 +0200 | Subject | Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates | From | Emanuele Giuseppe Esposito <> |
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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
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