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
