On Fri, Mar 10, 2023 at 04:22:45PM -0800, Sean Christopherson wrote:
> Disallow moving memslots if the VM has external page-track users, i.e. if
> KVMGT is being used to expose a virtual GPU to the guest, as KVM doesn't
> correctly handle moving memory regions.
>
> Note, this is potential ABI breakage! E.g. userspace could move regions
> that aren't shadowed by KVMGT without harming the guest. However, the
> only known user of KVMGT is QEMU, and QEMU doesn't move generic memory
> regions. KVM's own support for moving memory regions was also broken for
> multiple years (albeit for an edge case, but arguably moving RAM is
> itself an edge case), e.g. see commit edd4fa37baa6 ("KVM: x86: Allocate
> new rmap and large page tracking when moving memslot").
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
...
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 29dd6c97d145..47ac9291cd43 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12484,6 +12484,13 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
> struct kvm_memory_slot *new,
> enum kvm_mr_change change)
> {
> + /*
> + * KVM doesn't support moving memslots when there are external page
> + * trackers attached to the VM, i.e. if KVMGT is in use.
> + */
> + if (change == KVM_MR_MOVE && kvm_page_track_has_external_user(kvm))
> + return -EINVAL;
Hmm, will page track work correctly on moving memslots when there's no
external users?
in case of KVM_MR_MOVE,
kvm_prepare_memory_region(kvm, old, new, change)
|->kvm_arch_prepare_memory_region(kvm, old, new, change)
|->kvm_alloc_memslot_metadata(kvm, new)
|->memset(&slot->arch, 0, sizeof(slot->arch));
|->kvm_page_track_create_memslot(kvm, slot, npages)
The new->arch.arch.gfn_write_track will be fresh empty.
kvm_arch_commit_memory_region(kvm, old, new, change);
|->kvm_arch_free_memslot(kvm, old);
|->kvm_page_track_free_memslot(slot);
The old->arch.gfn_write_track is freed afterwards.
So, in theory, the new GFNs are not write tracked though the old ones are.
Is that acceptable for the internal page-track user?
> if (change == KVM_MR_CREATE || change == KVM_MR_MOVE) {
> if ((new->base_gfn + new->npages - 1) > kvm_mmu_max_gfn())
> return -EINVAL;
> --
> 2.40.0.rc1.284.g88254d51c5-goog
>
