Re: [PATCH] KVM: x86: Disallow EFER.LMSLE when EferLmsleUnsupported is set in guest CPUID

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 22, 2026, Jim Mattson wrote:
> On Tue, Sep 22, 2026 at 6:57 AM Sean Christopherson <seanjc@google.com> wrote:
> > Gah, I hate the negative "feature" bits, I've gotten completely turned around
> > multiple times trying to sort through this.
> 
> Me too!
> 
> > IIUC, you're trying to "fix" setups where host CPUID.EFER_LMSLE_MBZ=0, but userspace
> > configured guest CPUID.EFER_LMSLE_MBZ=1?  If so, then Sashiko is right, it needs to
> > be declared with EMULATED_F().  I would even argue that technically this isn't a KVM
> > bug, because KVM won't advertise EFER_LMSLE_MBZ in KVM_GET_SUPPORTED_CPUID on such
> > hosts.
> 
> I'm trying to make it possible to run a Rome vCPU on Milan or later.
> 
> SInce this is a negative feature bit, it must be retained if the host
> reports it. That blocks running a Rome vCPU on Milan or later.
> However, a defeatured Rome (which reports LMSLE_MBZ as 1) can be run

So, a partially defeatured Rome?  Because it sounds like the CPU reports LMSLE_MBZ=1,
but then still allows setting EFER.LMSLE=1?

> on Milan and later.
> 
> It's the same feature-hiding we do all of the time—just inverted.