Messages in this thread | | | Date | Mon, 19 Sep 2022 19:30:31 +0200 | From | David Hildenbrand <> | Subject | Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates |
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On 19.09.22 09:53, David Hildenbrand wrote: > 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. >
FWIW, I hacked up my RFC to perform atomic updates on any memslot transactions (not just resizes) where ranges do add overlap with ranges to remove.
https://github.com/davidhildenbrand/qemu/tree/memslot
I only performed simple boot check under x86-64 (where I can see region resizes) and some make checks -- pretty sure it has some rough edges; but should indicate what's possible and what the possible price might be. [one could wire up a new KVM ioctl and call it conditionally on support if really required]
-- Thanks,
David / dhildenb
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