linux-kernel - Re: [PATCH v2 2/2] KVM: X86: Fix loss of exception which has not yet injected

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Date:   Wed, 23 Aug 2017 14:27:28 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v2 2/2] KVM: X86: Fix loss of exception which has not yet
 injected

On 23/08/2017 12:23, Wanpeng Li wrote:
> @@ -6341,7 +6345,8 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
>  	int r;
>  
>  	/* try to reinject previous events if any */
> -	if (vcpu->arch.exception.pending) {
> +	if (vcpu->arch.exception.pending ||
> +		vcpu->arch.exception.injected) {
>  		trace_kvm_inj_exception(vcpu->arch.exception.nr,
>  					vcpu->arch.exception.has_error_code,
>  					vcpu->arch.exception.error_code);
> @@ -6357,6 +6362,11 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
>  		}
>  
>  		r = kvm_x86_ops->queue_exception(vcpu);
> +		if (r == 0 && vcpu->arch.exception.pending &&
> +			!vcpu->arch.exception.injected) {
> +			vcpu->arch.exception.pending = false;
> +			vcpu->arch.exception.injected = true;
> +		}
>  		return r;
>  	}
>  

There are some changes needed here:

- this "if" should check only .injected and call
kvm_x86_ops->queue_exception.  The "if" for .pending, which handles
rflags/dr7, assigns false to .pending and true to .injected, and also
calls kvm_x86_ops->queue_exception, should be after the call to
check_nested_events.

- in the call to inject_pending_event, the "else" should have a

	WARN_ON(vcpu->arch.exception.pending);

just for completeness.


Also, nested_vmx_check_exception has to be moved from
vmx_queue_exception to vmx_check_nested_events, so that
nested_run_pending is checked correctly.  Something like this:

        if (vcpu->arch.exception.pending &&
	    nested_vmx_check_exception(vcpu, &exit_qual)) {
		if (vmx->nested.nested_run_pending)
                        return -EBUSY;

		nested_vmx_inject_exception_vmexit(vcpu, exit_qual);
		return 0;
        }

Maybe you can have:

- patch 1 which I'll apply now

- patch 2 is the same as this one, with only the above changes to
inject_pending_event and the WARN_ON.

- patch 3 moves the nested_vmx_inject_exception_vmexit call from
nested_vmx_check_exception to vmx_queue_exception.

- patch 4 moves the code to vmx_check_nested_events and add the
nested_run_pending check, fixing the original bug.

As to GET/SET_VCPU_EVENTS, for now I would not do any change.  Instead,
we can do

	/*
	 * FIXME: pass injected and pending separately.  This is only
	 * needed for nested virtualization, whose state cannot be
	 * migrated yet.  For now we combine them just in case.
	 */
	events->exception.injected =
		(vcpu->arch.exception.pending ||
		 vcpu->arch.exception.injected) &&
		!kvm_exception_is_soft(vcpu->arch.exception.nr);
	}

and set_vcpu_events can just set .injected = false.  Separating the two
will need a separate capability and KVM_ENABLE_CAP.

Paolo

> 
> diff --git a/tools/arch/x86/include/uapi/asm/kvm.h b/tools/arch/x86/include/uapi/asm/kvm.h
> index c2824d0..7c55916 100644
> --- a/tools/arch/x86/include/uapi/asm/kvm.h
> +++ b/tools/arch/x86/include/uapi/asm/kvm.h
> @@ -296,9 +296,9 @@ struct kvm_reinject_control {
>  struct kvm_vcpu_events {
>  	struct {
>  		__u8 injected;
> +		__u8 pending;
>  		__u8 nr;
>  		__u8 has_error_code;
> -		__u8 pad;
>  		__u32 error_code;
>  	} exception;
>  	struct {

I think you don't need t