KVM: Protect all of kvm_vm_ioctl_create_vcpu() with kvm->lock

When creating a vCPU, don't drop kvm->lock to when doing the bulk of actual
vCPU creation, as allowing multiple vCPUs to be created in parallel adds
significant complexity in KVM (as evidenced by the many related bugs), and
all known VMMs fully serialize vCPU creation.  Remove all manually locking
of kvm->lock from kvm_arch_vcpu_{post,}create() for obvious reasons.

For many years, "everyone" has assumed that dropping kvm->lock was done for
performance reasons optimization, e.g. to allow userspace to create all
vCPUs concurrently for latency purposes.  But as above, no known VMM does
that.  Looking at the history of this code, before commit 11ec28047118
("KVM: Convert vm lock to a mutex"), kvm->lock was a spinlock.  I.e. KVM
*had* to drop kvm->lock when doing the bulk of vCPU creation, otherwise KVM
couldn't do normal memory allocations.  When kvm->lock got turned into a
mutex for unrelated reasons, no one took advantage updated of the change to
simplify vCPU creation.  And 19 years later, everyone just assumed that KVM
continued to deal with the complexity for performance reasons.

Furthermore, naively parallelizing vCPU creation in userspace is likely a
net negative due to the overheads of task creation.  Unless a VMM carefully
avoids the extra overhead related to parallelization, e.g. spawns each
vCPU's thread before creating the vCPU, creating vCPUs concurrently is a
net *negative* up until about ~64 vCPUs, after which the times are a wash.

The absolute speed of light _is_ faster if KVM doesn't hold kvm-lock, but
at vCPU counts of ~16 or less, it's probably in the noise when considering
total VM creation time, as the added latency is less than 1ms up until 16
or so vCPUs.

On top of all that, KVM has had a *lot* of fatal bugs (most often found by
syzkaller) related to vCPUs being created while trying to do per-VM
operations (basically, see every flow that locks all vCPUs).  I.e. the
parallel vCPU creation "support" is actively harmful as the only "use case"
is for misbehaving userspace to exploit KVM bugs.

Serializing vCPU creation will allow reverting commit 97d65b544f48 ("KVM:
Check for duplicate vcpu_id as early as possible"), which had "minor" math
error: the worst case scenario isn't "256 bytes per VM", it's "256 unsigned
longs per VM", i.e. 2048 bytes per VM, which doubles the size of each VM
and pushes several architectures into order-1 allocations.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Jean-Christophe Guillain <jean-christophe@guillain.net>
Tested-by: Naveen N Rao (AMD) <naveen@kernel.org>
Message-ID: <20260921174445.911676-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

0 files changed, 0 insertions, 0 deletions