Re: [PATCH] kvm: x86: rewrite kvm_spec_ctrl_valid_bits

Wanpeng Li
2020-07-07T08:56:45Z

On Tue, 7 Jul 2020 at 16:15, Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> Aren't you supposed to be on vacation? :-)

A long vacation, enjoy!

>
> On Tue, Jul 07, 2020 at 10:04:22AM +0200, Paolo Bonzini wrote:
> > On 07/07/20 08:11, 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.
>
> 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.