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.
