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

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Message-ID: <20220114100849.277c04ee@redhat.com>
Date:   Fri, 14 Jan 2022 10:08:49 +0100
From:   Igor Mammedov <imammedo@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.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

On Thu, 13 Jan 2022 20:00:08 +0000
Sean Christopherson <seanjc@...gle.com> 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?


> 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?
>