Re: [PATCH] x86/kvm: disable fast MMIO when running nested

Radim Krčmář
2018-01-25T14:16:35Z

2018-01-25 01:55-0800, Liran Alon:
> ----- vkuznets@redhat.com wrote:
> > 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 <vkuznets@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.
>
> If Intel manual doesn't mandate VM_EXIT_INSTRUCTION_LEN to be set in VMCS on EPT_MISCONFIG,
> I don't think we should do this on real-hardware as-well.

Neither do I, but you can see the last discussion on this topic,
https://patchwork.kernel.org/patch/9903811/. In short, we've agreed to
limit the hack to real hardware and wait for Intel or virtio changes.

Michael and Jason, any progress on implementing a fast virtio mechanism
that doesn't rely on undefined behavior?

(Encode writing instruction length into last 4 bits of MMIO address,
side-channel say that accesses to the MMIO area always use certain
instruction length, use hypercall, ...)

Thanks.