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

[Vitaly Kuznetsov]

Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:

> ----- 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.
>

Oh, I didn't know there was a story! I'll try EMULTYPE_SKIP, thanks!

-- 
  Vitaly