linux-kernel - Re: [PATCH 3/4] KVM: async_pf: Force a nested vmexit if the injected #PF is async

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Date:   Tue, 13 Jun 2017 20:55:23 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH 3/4] KVM: async_pf: Force a nested vmexit if the injected
 #PF is async_pf

2017-06-12 23:08-0700, Wanpeng Li:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> Add an async_page_fault field to vcpu->arch.exception to identify an async 
> page fault, and constructs the expected vm-exit information fields. Force 
> a nested VM exit from nested_vmx_check_exception() if the injected #PF 
> is async page fault.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> @@ -2422,13 +2422,28 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
>  static int nested_vmx_check_exception(struct kvm_vcpu *vcpu, unsigned nr)

This function could use the same treatment as vmx_queue_exception(), so
we are not mixing 'nr' with 'vcpu->arch.exception.*'.

>  {
>  	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> +	u32 intr_info = 0;
> +	unsigned long exit_qualification = 0;
>  
> -	if (!(vmcs12->exception_bitmap & (1u << nr)))
> +	if (!((vmcs12->exception_bitmap & (1u << nr)) ||
> +		(nr == PF_VECTOR && vcpu->arch.exception.async_page_fault)))
>  		return 0;
>  
> +	intr_info = nr | INTR_INFO_VALID_MASK;
> +	exit_qualification = vmcs_readl(EXIT_QUALIFICATION);

This part still uses EXIT_QUALIFICATION(), which means it is not general
and I think it would be nicer to just do simple special case on top:

	if (vcpu->arch.exception.async_page_fault) {
		vmcs_write32(VM_EXIT_INTR_ERROR_CODE, vcpu->arch.exception.error_code);
		nested_vmx_vmexit(vcpu, EXIT_REASON_EXCEPTION_NMI,
		                  PF_VECTOR | INTR_TYPE_HARD_EXCEPTION |
		                  INTR_INFO_DELIVER_CODE_MASK | INTR_INFO_VALID_MASK,
		                  vcpu->arch.cr2);
		return 1;
	}

Using vcpu->arch.cr2 is suspicious as VMX doesn't update CR2 on VM
exits;  isn't this going to change the CR2 visible in L2 guest after a
nested VM entry?

Btw. nested_vmx_check_exception() didn't support emulated exceptions at
all (it only passed through ones we got from hardware), or have I missed
something?

Thanks.