VFIO: KVM: x86: Drop kvm_arch_{start,end}_assignment()

Drop kvm_arch_{start,end}_assignment() and all associated code now that
KVM x86 no longer consumes assigned_device_count.  Tracking whether or not
a VFIO-assigned device is formally associated with a VM is fundamentally
flawed, as such an association is optional for general usage, i.e. is prone
to false negatives.  E.g. prior to commit 2edd9cb ("kvm: detect
assigned device via irqbypass manager"), device passthrough via VFIO would
fail to enable IRQ bypass if userspace omitted the formal VFIO<=>KVM
binding.

And device drivers that *need* the VFIO<=>KVM connection, e.g. KVM-GT,
shouldn't be relying on generic x86 tracking infrastructure.

Cc: Jim Mattson <jmattson@google.com>
Link: https://lore.kernel.org/r/20250523011756.3243624-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/include/asm/kvm_host.h

@@ -1381,8 +1381,6 @@ struct kvm_arch {
 
 #define __KVM_HAVE_ARCH_NONCOHERENT_DMA
 	atomic_t noncoherent_dma_count;
-#define __KVM_HAVE_ARCH_ASSIGNED_DEVICE
-	atomic_t assigned_device_count;
 	unsigned long nr_possible_bypass_irqs;
 
 #ifdef CONFIG_KVM_IOAPIC

arch/x86/kvm/x86.c

@@ -13438,24 +13438,6 @@ bool kvm_arch_can_dequeue_async_page_present(struct kvm_vcpu *vcpu)
 		return kvm_lapic_enabled(vcpu) && apf_pageready_slot_free(vcpu);
 }
 
-void kvm_arch_start_assignment(struct kvm *kvm)
-{
-	atomic_inc(&kvm->arch.assigned_device_count);
-}
-EXPORT_SYMBOL_GPL(kvm_arch_start_assignment);
-
-void kvm_arch_end_assignment(struct kvm *kvm)
-{
-	atomic_dec(&kvm->arch.assigned_device_count);
-}
-EXPORT_SYMBOL_GPL(kvm_arch_end_assignment);
-
-bool noinstr kvm_arch_has_assigned_device(struct kvm *kvm)
-{
-	return raw_atomic_read(&kvm->arch.assigned_device_count);
-}
-EXPORT_SYMBOL_GPL(kvm_arch_has_assigned_device);
-
 static void kvm_noncoherent_dma_assignment_start_or_stop(struct kvm *kvm)
 {
 	/*

include/linux/kvm_host.h

@@ -1690,24 +1690,6 @@ static inline bool kvm_arch_has_noncoherent_dma(struct kvm *kvm)
 	return false;
 }
 #endif
-#ifdef __KVM_HAVE_ARCH_ASSIGNED_DEVICE
-void kvm_arch_start_assignment(struct kvm *kvm);
-void kvm_arch_end_assignment(struct kvm *kvm);
-bool kvm_arch_has_assigned_device(struct kvm *kvm);
-#else
-static inline void kvm_arch_start_assignment(struct kvm *kvm)
-{
-}
-
-static inline void kvm_arch_end_assignment(struct kvm *kvm)
-{
-}
-
-static __always_inline bool kvm_arch_has_assigned_device(struct kvm *kvm)
-{
-	return false;
-}
-#endif
 
 static inline struct rcuwait *kvm_arch_vcpu_get_wait(struct kvm_vcpu *vcpu)
 {

virt/kvm/vfio.c

@@ -175,7 +175,6 @@ static int kvm_vfio_file_add(struct kvm_device *dev, unsigned int fd)
 	kvf->file = get_file(filp);
 	list_add_tail(&kvf->node, &kv->file_list);
 
-	kvm_arch_start_assignment(dev->kvm);
 	kvm_vfio_file_set_kvm(kvf->file, dev->kvm);
 	kvm_vfio_update_coherency(dev);
 
@@ -205,7 +204,6 @@ static int kvm_vfio_file_del(struct kvm_device *dev, unsigned int fd)
 			continue;
 
 		list_del(&kvf->node);
-		kvm_arch_end_assignment(dev->kvm);
 #ifdef CONFIG_SPAPR_TCE_IOMMU
 		kvm_spapr_tce_release_vfio_group(dev->kvm, kvf);
 #endif
@@ -336,7 +334,6 @@ static void kvm_vfio_release(struct kvm_device *dev)
 		fput(kvf->file);
 		list_del(&kvf->node);
 		kfree(kvf);
-		kvm_arch_end_assignment(dev->kvm);
 	}
 
 	kvm_vfio_update_coherency(dev);