[PATCH 4/5] Revert "KVM: Check for duplicate vcpu_id as early as possible"

[Sean Christopherson]

Now that KVM uses kvm_get_vcpu_by_id() to check for an existing vCPU ID
before doing any meaningful work, which was made possible by holding
kvm->lock for the entirety of vCPU creation, revert the now-redundant
"early" vCPU ID tracking.  The claims about the impact of kvm->vcpu_ids on
the memory footprint were a wee bit wrong: the worst case scenario isn't
256 bytes per VM, it's 256 "unsigned longs" per VM, i.e. 2048 bytes per VM.

Increasing the size of "struct kvm" by 2048 nearly doubled the total size
on many architectures, and tripped x86's KVM_SANITY_CHECK_VM_STRUCT_SIZE,
which was added to detect this *exact* scenario, where a single change
significantly increased the size of "struct kvm".  I.e. attempting to build
KVM with CONFIG_DEBUG_KERNEL=n fails on x86 (the build failures got missed
because all build bots apparently test only CONFIG_DEBUG_KERNEL=y kernels,
and maintainers' test flows were similarly lacking).

This reverts commit 97d65b544f48b2ee49f6aea32145e3e7969955dc.

Fixes: 97d65b544f48 ("KVM: Check for duplicate vcpu_id as early as possible")
Reported-by: Jean-Christophe Guillain <jean-christophe@xxxxxxxxxxxx>
Closes: https://lore.kernel.org/all/56a4bc35ee605588b7cc36c8e45c12b5f3b506cb.camel@xxxxxxxxxxxx
Reported-by: Paweł S <spawel523@xxxxxxxxx>
Closes: https://lore.kernel.org/all/CABD%3DWFOS4j4hDv%2BpW-eEM9HAM2q2GY_iYdAG%2BqvYcUEinUrcQQ@xxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 include/linux/kvm_host.h | 1 -
 virt/kvm/kvm_main.c      | 5 -----
 2 files changed, 6 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 03bfc92864b6..6aab167bf482 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -791,7 +791,6 @@ struct kvm {
 	/* The current active memslot set for each address space */
 	struct kvm_memslots __rcu *memslots[KVM_MAX_NR_ADDRESS_SPACES];
 	struct xarray vcpu_array;
-	DECLARE_BITMAP(vcpu_ids, KVM_MAX_VCPU_IDS);
 	/*
 	 * Protected by slots_lock, but can be read outside if an
 	 * incorrect answer is acceptable.
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d5524ac8c5cf..985af39b980a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4185,15 +4185,11 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
 	if (kvm_get_vcpu_by_id(kvm, id))
 		return -EEXIST;
 
-	if (WARN_ON_ONCE(test_bit(id, kvm->vcpu_ids)))
-		return -EEXIST;
-
 	r = kvm_arch_vcpu_precreate(kvm, id);
 	if (r)
 		return r;
 
 	kvm->created_vcpus++;
-	__set_bit(id, kvm->vcpu_ids);
 
 	vcpu = kmem_cache_zalloc(kvm_vcpu_cache, GFP_KERNEL_ACCOUNT);
 	if (!vcpu) {
@@ -4276,7 +4272,6 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
 	kmem_cache_free(kvm_vcpu_cache, vcpu);
 vcpu_decrement:
 	kvm->created_vcpus--;
-	__clear_bit(id, kvm->vcpu_ids);
 	return r;
 }
 
-- 
2.55.0.1032.g73a4cd73de-goog