Re: [PATCH v4 16/29] KVM: x86: Reject memslot MOVE operations if KVMGT is attached

[Like Xu]

On 2023/7/29 09:35, 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 KVMGT 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

This change breaks two kvm selftests:

- set_memory_region_test;
- memslot_perf_test;

Please help confirm if the tests/doc needs to be updated,
or if the assumption needs to be further clarified.

> 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").
>
> Reviewed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Tested-by: Yongwei Ma <yongwei.ma@xxxxxxxxx>
> 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 8c4d216e3b2b..f744682648e7 100644
> --- a/arch/x86/include/asm/kvm_page_track.h
> +++ b/arch/x86/include/asm/kvm_page_track.h
> @@ -75,4 +75,7 @@ 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 891e5cc52b45..e6de9638e560 100644
> --- a/arch/x86/kvm/mmu/page_track.c
> +++ b/arch/x86/kvm/mmu/page_track.c
> @@ -303,3 +303,8 @@ void kvm_page_track_flush_slot(struct kvm *kvm, struct kvm_memory_slot *slot)
>   			n->track_flush_slot(kvm, slot, n);
>   	srcu_read_unlock(&head->track_srcu, idx);
>   }
> +
> +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 059571d5abed..4394bb49051f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12606,6 +12606,13 @@ 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;