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

[Vitaly Kuznetsov]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> 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 cannot say I disagree)

>
> 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?

I see. Another approach would be to switch from the current allowlist
approach to a blocklist of things which we forbid to change
("MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, ...") after the
first KVM_RUN.

-- 
Vitaly