2018-01-24 16:12+0100, Vitaly Kuznetsov: > I was investigating an issue with seabios >= 1.10 which stopped working > for nested KVM on Hyper-V. The problem appears to be in > handle_ept_violation() function: when we do fast mmio we need to skip > the instruction so we do kvm_skip_emulated_instruction(). This, however, > depends on VM_EXIT_INSTRUCTION_LEN field being set correctly in VMCS. > However, this is not the case. > > Intel's manual doesn't mandate VM_EXIT_INSTRUCTION_LEN to be set when > EPT MISCONFIG occurs. While on real hardware it was observed to be set, > some hypervisors follow the spec and don't set it; we end up advancing > IP with some random value. > > I checked with Microsoft and they confirmed they don't fill > VM_EXIT_INSTRUCTION_LEN on EPT MISCONFIG. > > Fix the issue by disabling fast mmio when running nested. > > Signed-off-by: Vitaly Kuznetsov <vkuzn...@redhat.com> > --- > arch/x86/kvm/vmx.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > index c829d89e2e63..54afb446f38e 100644 > --- a/arch/x86/kvm/vmx.c > +++ b/arch/x86/kvm/vmx.c > @@ -6558,9 +6558,16 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu) > /* > * A nested guest cannot optimize MMIO vmexits, because we have an > * nGPA here instead of the required GPA. > + * Skipping instruction below depends on undefined behavior: Intel's > + * manual doesn't mandate VM_EXIT_INSTRUCTION_LEN to be set in VMCS > + * when EPT MISCONFIG occurs and while on real hardware it was observed > + * to be set, other hypervisors (namely Hyper-V) don't set it, we end > + * up advancing IP with some random value. Disable fast mmio when > + * running nested and keep it for real hardware in hope that > + * VM_EXIT_INSTRUCTION_LEN will always be set correctly. > */ > gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS); > - if (!is_guest_mode(vcpu) && > + if (!static_cpu_has(X86_FEATURE_HYPERVISOR) && !is_guest_mode(vcpu) &&
I realized that Paolo kept a minor optimization while getting rid of the undefined behavior (https://patchwork.kernel.org/patch/9903811/). Please do the same trick that signals kvm_io_bus_write() before going to x86_emulate_instruction(... EMULTYPE_SKIP ...), but add a branch to use kvm_skip_emulated_instruction() for bare-metal, thanks. > !kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) { > trace_kvm_fast_mmio(gpa); > return kvm_skip_emulated_instruction(vcpu); > -- > 2.14.3 >
