linux-kernel - Re: [PATCH] kvm: nVMX: fix entry with pending interrupt if APICv is enabled

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Date:   Wed, 3 Oct 2018 16:41:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Nikita Leshenko <nikita.leshchenko@...cle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Liran Alon <liran.alon@...cle.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] kvm: nVMX: fix entry with pending interrupt if APICv is
 enabled

On 03/10/2018 16:36, Nikita Leshenko wrote:
> On Wed, 2018-10-03 at 13:47 +0200, Paolo Bonzini wrote:
>> Commit b5861e5cf2fcf83031ea3e26b0a69d887adf7d21 introduced a check on
>> the interrupt-window and NMI-window CPU execution controls in order to
>> inject an external interrupt vmexit before the first guest instruction
>> executes.  However, when APIC virtualization is enabled the host does not
>> need a vmexit in order to inject an interrupt at the next interrupt window;
>> instead, it just places the interrupt vector in RVI and the processor will
>> inject it as soon as possible.  Therefore, on machines with APICv it is
>> not enough to check the CPU execution controls: the same scenario can also
>> happen if RVI>0.
>>
>> Fixes: b5861e5cf2fcf83031ea3e26b0a69d887adf7d21
>> Cc: Nikita Leshchenko <nikita.leshchenko@...cle.com>
>> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
>> Cc: Liran Alon <liran.alon@...cle.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>>  arch/x86/kvm/vmx.c | 16 +++++++++++-----
>>  1 file changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 6ef2d5b139b9..c0c7689f0049 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -10280,6 +10280,11 @@ static void vmx_hwapic_isr_update(struct kvm_vcpu *vcpu, int max_isr)
>>  	}
>>  }
>>  
>> +static u8 vmx_get_rvi(void)
>> +{
>> +	return vmcs_read16(GUEST_INTR_STATUS) & 0xff;
>> +}
>> +
>>  static void vmx_set_rvi(int vector)
>>  {
>>  	u16 status;
>> @@ -12593,10 +12598,13 @@ static int enter_vmx_non_root_mode(struct kvm_vcpu *vcpu, u32 *exit_qual)
>>  	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>>  	bool from_vmentry = !!exit_qual;
>>  	u32 dummy_exit_qual;
>> -	u32 vmcs01_cpu_exec_ctrl;
>> +	bool evaluate_pending_interrupts;
>>  	int r = 0;
>>  
>> -	vmcs01_cpu_exec_ctrl = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
>> +	evaluate_pending_interrupts = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL) &
>> +		(CPU_BASED_VIRTUAL_INTR_PENDING | CPU_BASED_VIRTUAL_NMI_PENDING);
>> +	if (enable_apicv && kvm_vcpu_apicv_active(vcpu))
>> +		evaluate_pending_interrupts |= vmx_get_rvi() > 0;
> 
> You should check for RVI > VPPR, similarly to how it is done in
> vmx_guest_apic_has_interrupt().

True, however that would only result in a spurious KVM_REQ_EVENT.
Unlike vmx_guest_apic_has_interrupt, we would have to check the TPR
shadow and SVI instead of the nested APIC page, so you'd have one more
VMREAD---and little benefit in the common case.

What do you think about adding a comment?

> Also, now that you introduced vmx_get_rvi(), it could be nice to use it
> in vmx_guest_apic_has_interrupt() as well.

True that.

Paolo

> Apart from that, looks good.
> 
> Reviewed-by: Nikita Leshenko <nikita.leshchenko@...cle.com>
> 
> Thanks,
> Nikita
> 
>>  
>>  	enter_guest_mode(vcpu);
>>  
>> @@ -12650,10 +12658,8 @@ static int enter_vmx_non_root_mode(struct kvm_vcpu *vcpu, u32 *exit_qual)
>>  	 * instead. Thus, we force L0 to perform pending event
>>  	 * evaluation by requesting a KVM_REQ_EVENT.
>>  	 */
>> -	if (vmcs01_cpu_exec_ctrl &
>> -		(CPU_BASED_VIRTUAL_INTR_PENDING | CPU_BASED_VIRTUAL_NMI_PENDING)) {
>> +	if (evaluate_pending_interrupts)
>>  		kvm_make_request(KVM_REQ_EVENT, vcpu);
>> -	}
>>  
>>  	/*
>>  	 * Note no nested_vmx_succeed or nested_vmx_fail here. At this point
>