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Date:   Tue, 27 Mar 2018 00:40:20 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH 2/2] KVM: VMX: Add Force Emulation Prefix for "emulate
 the next instruction"
Message-ID: <20180327044020.GB16974@char.us.oracle.com>
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On Mon, Mar 26, 2018 at 07:12:15PM -0700, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
>=20
> This patch introduces a Force Emulation Prefix (ud2a; .ascii "kvm") for=
=20
> "emulate the next instruction", the codes will be executed by emulator=20
> instead of processor, for testing purposes.

Can you expand a bit ? Why do you want this in KVM in the first place?
Should this be controlled by a boolean parameter?=20
>=20
> A testcase here:
>=20
> #include <stdio.h>
> #include <string.h>
>   =20
> #define HYPERVISOR_INFO 0x40000000
>   =20
> #define CPUID(idx, eax, ebx, ecx, edx)\
>     asm volatile (\
>     "ud2a; .ascii \"kvm\"; 1: cpuid" \
>     :"=3Db" (*ebx), "=3Da" (*eax),"=3Dc" (*ecx), "=3Dd" (*edx)\
>         :"0"(idx) ); =20
>   =20
> void main() =20
> { =20
> 	unsigned int eax,ebx,ecx,edx; =20
> 	char string[13]; =20
>   =20
> 	CPUID(HYPERVISOR_INFO, &eax, &ebx, &ecx, &edx); =20
> 	*(unsigned int *)(string+0) =3D ebx; =20
> 	*(unsigned int *)(string+4) =3D ecx; =20
> 	*(unsigned int *)(string+8) =3D edx; =20
>   =20
> 	string[12] =3D 0; =20
> 	if (strncmp(string, "KVMKVMKVM\0\0\0",12) =3D=3D 0)
> 		printf("kvm guest\n"); =20
> 	else =20
> 		printf("bare hardware\n"); =20
> }
>=20
> Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Kr=C4=8Dm=C3=A1=C5=99 <rkrcmar@redhat.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/vmx.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>=20
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 0f99833..90abed8 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -108,6 +108,9 @@ module_param_named(enable_shadow_vmcs, enable_shado=
w_vmcs, bool, S_IRUGO);
>  static bool __read_mostly nested =3D 0;
>  module_param(nested, bool, S_IRUGO);
> =20
> +static bool __read_mostly fep =3D 0;
> +module_param(fep, bool, S_IRUGO);
> +
>  static u64 __read_mostly host_xss;
> =20
>  static bool __read_mostly enable_pml =3D 1;
> @@ -6218,8 +6221,21 @@ static int handle_machine_check(struct kvm_vcpu =
*vcpu)
>  static int handle_ud(struct kvm_vcpu *vcpu)
>  {
>  	enum emulation_result er;
> +	int emulation_type =3D EMULTYPE_TRAP_UD;
> +
> +	if (fep) {
> +		char sig[5]; /* ud2; .ascii "kvm" */
> +		struct x86_exception e;

Don't you want to do =3D { };
to memset it?
> +
> +		kvm_read_guest_virt(&vcpu->arch.emulate_ctxt,
> +				kvm_get_linear_rip(vcpu), sig, sizeof(sig), &e);
> +		if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) =3D=3D 0) {
> +			emulation_type =3D 0;
> +			kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
> +		}
> +	}
> =20
> -	er =3D emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
> +	er =3D emulate_instruction(vcpu, emulation_type);
>  	if (er =3D=3D EMULATE_USER_EXIT)
>  		return 0;
>  	if (er !=3D EMULATE_DONE)
> --=20
> 2.7.4
>=20