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@xxxxxxxxxx/ "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
