linux-kernel - Re: [PATCH 5/5] KVM: VMX: Handle RFLAGS.TF in skip_emulated

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Date:   Mon, 28 Nov 2016 12:42:05 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Kyle Huey <me@...ehuey.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Joerg Roedel <joro@...tes.org>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] KVM: VMX: Handle RFLAGS.TF in
 skip_emulated_instruction



On 28/11/2016 05:18, Kyle Huey wrote:
> +
> +	if (unlikely(vmx_get_rflags(vcpu) & X86_EFLAGS_TF)) {
> +		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> +			vcpu->run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 |
> +						    DR6_RTM;
> +			vcpu->run->debug.arch.pc = vcpu->arch.singlestep_rip;
> +			vcpu->run->debug.arch.exception = DB_VECTOR;
> +			vcpu->run->exit_reason = KVM_EXIT_DEBUG;
> +			return 0;
> +		}
> +
> +		/*
> +		 * "Certain debug exceptions may clear bit 0-3.  The
> +		 * remaining contents of the DR6 register are never
> +		 * cleared by the processor".
> +		 */
> +		vcpu->arch.dr6 &= ~15;
> +		vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
> +		kvm_queue_exception(vcpu, DB_VECTOR);
> +	}

This code is pretty much the same as kvm_vcpu_check_singlestep.  Let's 
not duplicate the code and implement skip_emulated_instruction can be
implemented in x86.c, like

	unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
	int r = EMULATE_DONE;

	/* This would be the no_trap variant */
	kvm_x86_ops->skip_emulated_instruction(vcpu);
	kvm_vcpu_check_singlestep(vcpu, rflags, &r);
	return r == EMULATE_DONE;

(because x86.c/vmx.c/svm.c are separate modules, when moving the function
to x86.c you should rename it to kvm_skip_emulated_instruction).

Paolo