Re: [PATCH 0/5] KVM: Serialize vCPU creation and revert vcpu_ids tracking

[Paolo Bonzini]

On 9/21/26 15:57, Thorsten Leemhuis wrote:
> On 9/14/26 20:12, Sean Christopherson wrote:
> > Serialize vCPU creation by holding kvm->lock for the entirety of
> > kvm_vm_ioctl_create_vcpu(), and then revert the now-redundant tracking adding
> > by commit 97d65b544f48 ("KVM: Check for duplicate vcpu_id as early as
> > possible").  I botched the math when justifying the vcpu_ids tracking; it's not
> > an extra 256 bytes, it's an extra 2048 bytes.  Roughly doubling the size of
> > "struct kvm" tripped x86's KVM_SANITY_CHECK_VM_STRUCT_SIZE, and obviously isn't
> > something we want to do in general.
>
> Hi Sean! Thx for handling this and sorry for pestering you, but I have
> to ask, as this fixes a regression that was already reported two times
> and introduced this cycle:
>
> What's the plan to merge this? Is this aimed for the current or the next
> merge window? Ciao, Thorsten

For this one, but after fixing the PPC and s390 cases.  Sean, if you're 
busy I can send the v2 myself.

Paolo

> > The TL;DR of why it's a-ok to serialize vCPU creation is that no VMM actually
> > does parallel vCPU creation.  As with so many things, KVM's current behavior is
> > the result of decades-old cruft, not intentional, deliberate design.
> >
> > Patch 1 is a tangentially related bug fix; I included it here because holding
> > kvm->lock for all of vCPU creation allows WARNing if KVM attempts to lock all
> > vCPUs if vCPU creation is in-progress (the caller is must hold kvm->lock).
> >
> > Sean Christopherson (5):
> >    KVM: Reject attempts to lock all vCPUs if vCPU creation is in-progress
> >    KVM: Protect all of kvm_vm_ioctl_create_vcpu() with kvm->lock
> >    KVM: Move check for existing vCPU ID to the top of vCPU creation
> >    Revert "KVM: Check for duplicate vcpu_id as early as possible"
> >    KVM: WARN if vCPU creation is in-progress when locking all vCPUs
> >
> >   arch/x86/kvm/svm/sev.c   | 10 ----------
> >   arch/x86/kvm/vmx/tdx.c   |  5 -----
> >   include/linux/kvm_host.h |  1 -
> >   virt/kvm/kvm_main.c      | 35 +++++++++++------------------------
> >   4 files changed, 11 insertions(+), 40 deletions(-)
> >
> >
> > base-commit: d599822bdb66aeec5ec76297b0fc6efaaeefe07c