From 700f8d24436c45cb3cc8956f35c90a1b6a6487c2 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Mon, 21 Sep 2026 10:44:44 -0700 Subject: Revert "KVM: Check for duplicate vcpu_id as early as possible" MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Closes: https://lore.kernel.org/all/56a4bc35ee605588b7cc36c8e45c12b5f3b506cb.camel@guillain.net Reported-by: Paweł S Closes: https://lore.kernel.org/all/CABD%3DWFOS4j4hDv%2BpW-eEM9HAM2q2GY_iYdAG%2BqvYcUEinUrcQQ@mail.gmail.com Tested-by: Jean-Christophe Guillain Signed-off-by: Sean Christopherson Tested-by: Naveen N Rao (AMD) Message-ID: <20260921174445.911676-7-seanjc@google.com> Signed-off-by: Paolo Bonzini --- include/linux/kvm_host.h | 1 - 1 file changed, 1 deletion(-) (limited to 'include/linux') diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index cf7fe835c4ad..c844def6dd95 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. -- cgit v1.2.3