linux-kernel - Re: [PATCH 1/4] KVM: x86: pending exceptions must not be blocked by an injected event

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Date:   Thu, 1 Apr 2021 19:05:37 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Joerg Roedel <joro@...tes.org>,
        Ingo Molnar <mingo@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 1/4] KVM: x86: pending exceptions must not be blocked by
 an injected event

On 01/04/21 16:38, Maxim Levitsky wrote:
> Injected interrupts/nmi should not block a pending exception,
> but rather be either lost if nested hypervisor doesn't
> intercept the pending exception (as in stock x86), or be delivered
> in exitintinfo/IDT_VECTORING_INFO field, as a part of a VMexit
> that corresponds to the pending exception.
> 
> The only reason for an exception to be blocked is when nested run
> is pending (and that can't really happen currently
> but still worth checking for).
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>

This patch would be an almost separate bugfix, right?  I am going to 
queue this, but a confirmation would be helpful.

Paolo

> ---
>   arch/x86/kvm/svm/nested.c |  8 +++++++-
>   arch/x86/kvm/vmx/nested.c | 10 ++++++++--
>   2 files changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 8523f60adb92..34a37b2bd486 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1062,7 +1062,13 @@ static int svm_check_nested_events(struct kvm_vcpu *vcpu)
>   	}
>   
>   	if (vcpu->arch.exception.pending) {
> -		if (block_nested_events)
> +		/*
> +		 * Only a pending nested run can block a pending exception.
> +		 * Otherwise an injected NMI/interrupt should either be
> +		 * lost or delivered to the nested hypervisor in the EXITINTINFO
> +		 * vmcb field, while delivering the pending exception.
> +		 */
> +		if (svm->nested.nested_run_pending)
>                           return -EBUSY;
>   		if (!nested_exit_on_exception(svm))
>   			return 0;
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index fd334e4aa6db..c3ba842fc07f 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3806,9 +3806,15 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>   
>   	/*
>   	 * Process any exceptions that are not debug traps before MTF.
> +	 *
> +	 * Note that only a pending nested run can block a pending exception.
> +	 * Otherwise an injected NMI/interrupt should either be
> +	 * lost or delivered to the nested hypervisor in the IDT_VECTORING_INFO,
> +	 * while delivering the pending exception.
>   	 */
> +
>   	if (vcpu->arch.exception.pending && !vmx_pending_dbg_trap(vcpu)) {
> -		if (block_nested_events)
> +		if (vmx->nested.nested_run_pending)
>   			return -EBUSY;
>   		if (!nested_vmx_check_exception(vcpu, &exit_qual))
>   			goto no_vmexit;
> @@ -3825,7 +3831,7 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>   	}
>   
>   	if (vcpu->arch.exception.pending) {
> -		if (block_nested_events)
> +		if (vmx->nested.nested_run_pending)
>   			return -EBUSY;
>   		if (!nested_vmx_check_exception(vcpu, &exit_qual))
>   			goto no_vmexit;
>