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

[Sean Christopherson]

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.

I also don't love that the set of volatile entries is nothing more than "this is
what QEMU needs today".  There's no architectural justification, and the few cases
that do architecturally allow CPUID bits to change are disallowed.  E.g. OSXSAVE,
MONITOR/MWAIT, CPUID.0x12.EAX.SGX1 are all _architecturally_ defined scenarios
where CPUID can change, yet none of those appear in this list.  Some of those are
explicitly handled by KVM (runtime CPUID updates), but why should it be illegal
for userspace to intercept writes to MISC_ENABLE and do its own CPUID emulation?