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

[Yan Zhao <yan.y.zhao@intel.com>]

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@google.com>
...
> 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
>