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Subject: Re: [PATCH v5 2/2] KVM: X86: Add Force Emulation Prefix for "emulate
 the next instruction"
To:     Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Liran Alon <liran.alon@oracle.com>
References: <1522798129-14588-1-git-send-email-wanpengli@tencent.com>
 <1522798129-14588-3-git-send-email-wanpengli@tencent.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <48d06146-f508-e883-72cb-881e043f9c72@redhat.com>
Date:   Wed, 4 Apr 2018 13:59:13 +0200
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> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1eb495e..a55ecef 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -146,6 +146,9 @@ bool __read_mostly enable_vmware_backdoor = false;
>  module_param(enable_vmware_backdoor, bool, S_IRUGO);
>  EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
>  
> +static bool __read_mostly force_emulation_prefix = false;
> +module_param(force_emulation_prefix, bool, S_IRUGO);
> +
>  #define KVM_NR_SHARED_MSRS 16
>  
>  struct kvm_shared_msrs_global {
> @@ -4844,6 +4847,21 @@ int handle_ud(struct kvm_vcpu *vcpu)
>  {
>  	enum emulation_result er;
>  
> +	if (force_emulation_prefix) {
> +		char sig[5]; /* ud2; .ascii "kvm" */
> +		struct x86_exception e;
> +
> +		if (kvm_read_guest_virt(&vcpu->arch.emulate_ctxt,
> +				kvm_get_linear_rip(vcpu), sig, sizeof(sig), &e))
> +			goto emulate_ud;
> +
> +		if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
> +			kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
> +			return emulate_instruction(vcpu, 0) == EMULATE_DONE;

What if we would have an invalid instruction here? Shouldn't you handle
the emulate_instruction() like below?
(e.g. keep a variable with the emulation type (0 vs EMULTYPE_TRAP_UD)
and reuse emulate_ud below)

> +		}
> +	}
> +
> +emulate_ud:
>  	er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
>  	if (er == EMULATE_USER_EXIT)
>  		return 0;
> 


-- 

Thanks,

David / dhildenb