Re: [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update

[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!

Does the invalidation already free up memslot metadata (especially the 
rmaps) or will we end up temporarily allocating twice the memslot metadata?

--
Thanks,

David / dhildenb