Re: [PATCH 07/54] KVM: x86: Alert userspace that KVM_SET_CPUID{,2} after KVM_RUN is broken

[Jim Mattson]

On Wed, Jun 23, 2021 at 10:11 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 23/06/21 19:00, Jim Mattson wrote:
> > On Wed, Jun 23, 2021 at 7:16 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> >>
> >> On 22/06/21 19:56, Sean Christopherson wrote:
> >>> +     /*
> >>> +      * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
> >>> +      * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
> >>> +      * tracked in kvm_mmu_page_role.  As a result, KVM may miss guest page
> >>> +      * faults due to reusing SPs/SPTEs.  Alert userspace, but otherwise
> >>> +      * sweep the problem under the rug.
> >>> +      *
> >>> +      * KVM's horrific CPUID ABI makes the problem all but impossible to
> >>> +      * solve, as correctly handling multiple vCPU models (with respect to
> >>> +      * paging and physical address properties) in a single VM would require
> >>> +      * tracking all relevant CPUID information in kvm_mmu_page_role.  That
> >>> +      * is very undesirable as it would double the memory requirements for
> >>> +      * gfn_track (see struct kvm_mmu_page_role comments), and in practice
> >>> +      * no sane VMM mucks with the core vCPU model on the fly.
> >>> +      */
> >>> +     if (vcpu->arch.last_vmentry_cpu != -1)
> >>> +             pr_warn_ratelimited("KVM: KVM_SET_CPUID{,2} after KVM_RUN may cause guest instability\n");
> >>
> >> Let's make this even stronger and promise to break it in 5.16.
> >>
> >> Paolo
> >
> > Doesn't this fall squarely into kvm's philosophy of "we should let
> > userspace shoot itself in the foot wherever possible"? I thought we
> > only stepped in when host stability was an issue.
> >
> > I'm actually delighted if this is a sign that we're rethinking that
> > philosophy. I'd just like to hear someone say it.
>
> Nah, that's not the philosophy.  The philosophy is that covering all
> possible ways for userspace to shoot itself in the foot is impossible.
>
> However, here we're talking about 2 lines of code (thanks also to your
> patches that add last_vmentry_cpu for completely unrelated reasons) to
> remove a whole set of bullet/foot encounters.

What about the problems that arise when we have different CPUID tables
for different vCPUs in the same VM? Can we just replace this
hole-in-foot inducing ioctl with a KVM_VM_SET_CPUID ioctl on the VM
level that has to be called before any vCPUs are created?