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Date: Wed, 5 Feb 2020 21:05:19 -0800 From: Sean Christopherson <sean.j.christopherson@...el.com> To: Peter Xu <peterx@...hat.com> Cc: Paolo Bonzini <pbonzini@...hat.com>, Paul Mackerras <paulus@...abs.org>, Christian Borntraeger <borntraeger@...ibm.com>, Janosch Frank <frankja@...ux.ibm.com>, David Hildenbrand <david@...hat.com>, Cornelia Huck <cohuck@...hat.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, Jim Mattson <jmattson@...gle.com>, Joerg Roedel <joro@...tes.org>, Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>, Julien Thierry <julien.thierry.kdev@...il.com>, Suzuki K Poulose <suzuki.poulose@....com>, linux-mips@...r.kernel.org, kvm@...r.kernel.org, kvm-ppc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org, Christoffer Dall <christoffer.dall@....com>, Philippe Mathieu-Daudé <f4bug@...at.org> Subject: Re: [PATCH v5 01/19] KVM: x86: Allocate new rmap and large page tracking when moving memslot On Wed, Feb 05, 2020 at 09:58:58PM -0500, Peter Xu wrote: > On Wed, Feb 05, 2020 at 06:17:15PM -0800, Sean Christopherson wrote: > > On Wed, Feb 05, 2020 at 09:00:31PM -0500, Peter Xu wrote: > > > On Wed, Feb 05, 2020 at 03:55:33PM -0800, Sean Christopherson wrote: > > > > On Wed, Feb 05, 2020 at 04:49:52PM -0500, Peter Xu wrote: > > > > > Instead of calling kvm_arch_create_memslot() explicitly again here, > > > > > can it be replaced by below? > > > > > > > > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > > > > index 72b45f491692..85a7b02fd752 100644 > > > > > --- a/virt/kvm/kvm_main.c > > > > > +++ b/virt/kvm/kvm_main.c > > > > > @@ -1144,7 +1144,7 @@ int __kvm_set_memory_region(struct kvm *kvm, > > > > > new.dirty_bitmap = NULL; > > > > > > > > > > r = -ENOMEM; > > > > > - if (change == KVM_MR_CREATE) { > > > > > + if (change == KVM_MR_CREATE || change == KVM_MR_MOVE) { > > > > > new.userspace_addr = mem->userspace_addr; > > > > > > > > > > if (kvm_arch_create_memslot(kvm, &new, npages)) > > > > > > > > No, because other architectures don't need to re-allocate new metadata on > > > > MOVE and rely on __kvm_set_memory_region() to copy @arch from old to new, > > > > e.g. see kvmppc_core_create_memslot_hv(). > > > > > > Yes it's only required in x86, but iiuc it also will still work for > > > ppc? Say, in that case ppc won't copy @arch from old to new, and > > > kvmppc_core_free_memslot_hv() will free the old, however it should > > > still work. > > > > No, calling kvm_arch_create_memslot() for MOVE will result in PPC leaking > > memory due to overwriting slot->arch.rmap with a new allocation. > > Why? For the MOVE case, kvm_arch_create_memslot() will create a new > rmap for the "new" memslot. If the whole procedure succeeded, > kvm_free_memslot() will free the old rmap. If it failed, > kvm_free_memslot() will free the new rmap if !NULL. Looks fine? Oh, I see what you're suggesting. Please god no. This is a bug fix that needs to be backported to stable. Arbitrarily changing PPC behavior is a bad idea, especially since I don't know squat about the PPC rmap behavior. If it happens to fix a PPC rmap bug, then PPC should get an explicit fix. If it's not a bug fix, then at best it is a minor performance hit due to an extra allocation and the need to refill the rmap. Worst case scenario it breaks PPC. And unless this were a temporary change, which would be silly, I would have to carry forward the change into "KVM: PPC: Move memslot memory allocation into prepare_memory_region()", and again, I don't know squat about PPC. I also don't want to effectively introduce a misnamed function, even if only temporarily, e.g. it's kvm_arch_create_memslot(), not kvm_arch_create_or_move_memslot(), because the whole flow gets reworked a few patches later.
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