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

[Paolo Bonzini]

----- Original Message -----
> From: "Vitaly Kuznetsov" <vkuznets@xxxxxxxxxx>
> To: kvm@xxxxxxxxxxxxxxx
> Cc: x86@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, "Paolo Bonzini" <pbonzini@xxxxxxxxxx>, "Radim KrÄmÃÅ"
> <rkrcmar@xxxxxxxxxx>
> Sent: Wednesday, January 24, 2018 4:12:34 PM
> Subject: [PATCH] x86/kvm: disable fast MMIO when running nested
> 
> 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@xxxxxxxxxx>
> ---
>  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) &&
>  	    !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

Vitaly,

could you base the X86_FEATURE_HYPERVISOR case on the patch at
https://patchwork.kernel.org/patch/9903811/?

By using EMULTYPE_SKIP, the eventfd is signaled before entering the
emulator and the impact on latency is small.

Thanks,

Paolo