linux-kernel - Re: [PATCH 0/5] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM

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Date:   Fri, 14 Jan 2022 09:12:37 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] KVM: x86: Partially allow KVM_SET_CPUID{,2} after
 KVM_RUN for CPU hotplug

Sean Christopherson <seanjc@...gle.com> 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