Re: [PATCH 0/5] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug

[Sean Christopherson]

On Fri, Jan 14, 2022, Igor Mammedov wrote:
> On Thu, 13 Jan 2022 20:00:08 +0000
> Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> 
> > On Thu, Jan 13, 2022, Vitaly Kuznetsov wrote:
> > > Recently, KVM made it illegal to change CPUID after KVM_RUN but
> > > unfortunately this change is not fully compatible with existing VMMs.
> > > In particular, QEMU reuses vCPU fds for CPU hotplug after unplug and it
> > > calls KVM_SET_CPUID2. Relax the requirement by implementing an allowlist
> > > of entries which are allowed to change.  
> > 
> > Honestly, I'd prefer we give up and just revert feb627e8d6f6 ("KVM: x86: Forbid
> > KVM_SET_CPUID{,2} after KVM_RUN").  Attempting to retroactively restrict the
> > existing ioctls is becoming a mess, and I'm more than a bit concerned that this
> > will be a maintenance nightmare in the future, without all that much benefit to
> > anyone.
> 
> in 63f5a1909f9 ("KVM: x86: Alert userspace that KVM_SET_CPUID{,2} after KVM_RUN is broken")
> you mention heterogeneous configuration, and that implies that
> a userspace (not upstream qemu today) might attempt to change CPUID
> and that would be wrong. Do we still care about that?

We still care, and I really do like the idea in theory, but if we're stuck choosing
between taking on a pile of ugly code in KVM and letting userspace shoot themselves
in the foot, I choose the latter :-)