Messages in this thread | | | Date | Tue, 27 Sep 2022 11:32:43 +0200 | Subject | Re: [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update | From | Emanuele Giuseppe Esposito <> |
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Am 27/09/2022 um 11:22 schrieb David Hildenbrand: > On 27.09.22 10:35, Emanuele Giuseppe Esposito wrote: >> >> >> Am 27/09/2022 um 09:46 schrieb David Hildenbrand: >>> On 09.09.22 12:45, Emanuele Giuseppe Esposito wrote: >>>> When kvm_vm_ioctl_set_memory_region_list() is invoked, we need >>>> to make sure that all memslots are updated in the inactive list >>>> and then swap (preferreably only once) the lists, so that all >>>> changes are visible immediately. >>>> >>>> The only issue is that DELETE and MOVE need to perform 2 swaps: >>>> firstly replace old memslot with invalid, and then remove invalid. >>>> >>> >>> I'm curious, how would a resize (grow/shrink) or a split be handled? >>> >> >> There are only 4 operations possible in KVM: KVM_MR_{DELETE, MOVE, >> CREATE, FLAGS_ONLY}. >> >> A resize should be implemented in QEMU as DELETE+CREATE. >> >> Therefore a resize on memslot X will be implemented as: >> First pass on the userspace operations: >> invalidate memslot X; >> swap_memslot_list(); // NOW it is visible to the guest >> >> What guest sees: memslot X is invalid, so MMU keeps retrying the page >> fault >> >> Second pass: >> create new memslot X >> delete old memslot X > > Thanks a lot for the very nice explanation!
Anytime :)
> Does the invalidation already free up memslot metadata (especially the > rmaps) or will we end up temporarily allocating twice the memslot metadata? >
Invalidation creates a new temporary identical memslot, I am not sure about the rmaps. It is anyways the same code as it was done before and if I understand correctly, a new slot is required to keep the old intact, in case something goes wrong and we need to revert.
Thanks, Emanuele
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