Messages in this thread | | | Date | Mon, 19 Sep 2022 09:53:29 +0200 | Subject | Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates | From | David Hildenbrand <> |
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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
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