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

[Sean Christopherson]

On Wed, Aug 30, 2023, Like Xu wrote:
> 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;

It shoudn't.  As of this patch, KVM doesn't register itself as a page-track user,
i.e. KVMGT is the only remaining caller to kvm_page_track_register_notifier().
Unless I messed up, the only way kvm_page_track_has_external_user() can return
true is if KVMGT is attached to the VM.  The selftests most definitely don't do
anything with KVMGT, so I don't see how they can fail.

Are you seeing actually failures?

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

What assumption?

> > 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;