Re: [PATCH v2 14/27] KVM: x86: Reject memslot MOVE operations if KVMGT is attached

[Yan Zhao]

Reviewed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>

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>
> ---
>  arch/x86/include/asm/kvm_page_track.h | 3 +++
>  arch/x86/kvm/mmu/page_track.c         | 5 +++++
>  arch/x86/kvm/x86.c                    | 7 +++++++
>  3 files changed, 15 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
> index 0d65ae203fd6..6a287bcbe8a9 100644
> --- a/arch/x86/include/asm/kvm_page_track.h
> +++ b/arch/x86/include/asm/kvm_page_track.h
> @@ -77,4 +77,7 @@ kvm_page_track_unregister_notifier(struct kvm *kvm,
>  void kvm_page_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,
>  			  int bytes);
>  void kvm_page_track_flush_slot(struct kvm *kvm, struct kvm_memory_slot *slot);
> +
> +bool kvm_page_track_has_external_user(struct kvm *kvm);
> +
>  #endif
> diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
> index 39a0863af8b4..1cfc0a0ccc23 100644
> --- a/arch/x86/kvm/mmu/page_track.c
> +++ b/arch/x86/kvm/mmu/page_track.c
> @@ -321,3 +321,8 @@ enum pg_level kvm_page_track_max_mapping_level(struct kvm *kvm, gfn_t gfn,
>  	return max_level;
>  }
>  EXPORT_SYMBOL_GPL(kvm_page_track_max_mapping_level);
> +
> +bool kvm_page_track_has_external_user(struct kvm *kvm)
> +{
> +	return hlist_empty(&kvm->arch.track_notifier_head.track_notifier_list);
> +}
> 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;
> +
>  	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
>