linux-kernel - Re: [PATCH 2/4] KVM: nSVM: propagate the NPF EXITINFO to the guest

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Date:	Tue, 2 Sep 2014 18:33:44 +0200
From:	Joerg Roedel <jroedel@...e.de>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, agraf@...e.de,
	valentine.sinitsyn@...il.com, jan.kiszka@...mens.com,
	gleb@...udius-systems.com, avi@...udius-systems.com
Subject: Re: [PATCH 2/4] KVM: nSVM: propagate the NPF EXITINFO to the guest

Ah, here you add emulation of these bits.

On Tue, Sep 02, 2014 at 05:13:48PM +0200, Paolo Bonzini wrote:
> This is similar to what the EPT code does with the exit qualification.
> This allows the guest to see a valid value for bits 33:32.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/paging_tmpl.h |  6 ++++++
>  arch/x86/kvm/svm.c         | 26 ++++++++++++++++++++++----
>  2 files changed, 28 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 410776528265..99d4c4e836a0 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -322,8 +322,14 @@ retry_walk:
>  
>  		real_gfn = mmu->translate_gpa(vcpu, gfn_to_gpa(table_gfn),
>  					      PFERR_USER_MASK|PFERR_WRITE_MASK);
> +
> +		/*
> +		 * Can this happen (except if the guest is playing TOCTTOU games)?
> +		 * We should have gotten a nested page fault on table_gfn instead.
> +		 */

Comment is true, but doesn't make the check below obsolete, no?

>  		if (unlikely(real_gfn == UNMAPPED_GVA))
>  			goto error;
> @@ -1974,10 +1974,28 @@ static void nested_svm_inject_npf_exit(struct kvm_vcpu *vcpu,
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
> -	svm->vmcb->control.exit_code = SVM_EXIT_NPF;
> -	svm->vmcb->control.exit_code_hi = 0;
> -	svm->vmcb->control.exit_info_1 = fault->error_code;
> -	svm->vmcb->control.exit_info_2 = fault->address;
> +	/*
> +	 * We can keep the value that the processor stored in the VMCB,
> +	 * but make up something sensible if we hit the WARN.
> +	 */
> +	if (WARN_ON(svm->vmcb->control.exit_code != SVM_EXIT_NPF)) {
> +		svm->vmcb->control.exit_code = SVM_EXIT_NPF;
> +		svm->vmcb->control.exit_code_hi = 0;
> +		svm->vmcb->control.exit_info_1 = (1ULL << 32);
> +		svm->vmcb->control.exit_info_2 = fault->address;
> +	}

Its been a while since I looked into this, but is an injected NPF exit
always the result of a real NPF exit? How about an io-port emulated on
L1 but passed through to L2 by the nested hypervisor. On emulation of
INS or OUTS, KVM would need to read/write to an L2 address space, maybe
causing NPF faults to be injected. In this case an IOIO exit would cause
an injected NPF exit for L1.


	Joerg

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