Re: [PATCH] kvm: x86: rewrite kvm_spec_ctrl_valid_bits

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Jul 07, 2020 at 10:17:14AM +0200, Paolo Bonzini wrote:
> On 07/07/20 10:14, Sean Christopherson wrote:
> >>> One oddity with this whole thing is that by passing through the MSR, KVM is
> >>> allowing the guest to write bits it doesn't know about, which is definitely
> >>> not normal.  It also means the guest could write bits that the host VMM
> >>> can't.
> >> That's true.  However, the main purpose of the kvm_spec_ctrl_valid_bits
> >> check is to ensure that host-initiated writes are valid; this way, you
> >> don't get a #GP on the next vmentry's WRMSR to MSR_IA32_SPEC_CTRL.
> >> Checking the guest CPUID bit is not even necessary.
> > Right, what I'm saying is that rather than try and decipher specs to
> > determine what bits are supported, just throw the value at hardware and
> > go from there.  That's effectively what we end up doing for the guest writes
> > anyways.
> 
> Yes, it would prevent the #GP.
> 
> > Actually, the current behavior will break migration if there are ever legal
> > bits that KVM doesn't recognize, e.g. guest writes a value that KVM doesn't
> > allow and then migration fails when the destination tries to stuff the value
> > into KVM.
> 
> Yes, unfortunately migration would also be broken if the target (and the
> guest CPUID) is an older CPU.  But that's not something we can fix
> without trapping all writes which would be unacceptably slow.

Ah, true, the guest would need to be setting bits that weren't enumerated
to it.