linux-kernel - Re: [PATCH 2/2] KVM: VMX: Add Force Emulation Prefix for "emulate the next instruction"

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Date:   Tue, 27 Mar 2018 00:40:20 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH 2/2] KVM: VMX: Add Force Emulation Prefix for "emulate
 the next instruction"

On Mon, Mar 26, 2018 at 07:12:15PM -0700, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> This patch introduces a Force Emulation Prefix (ud2a; .ascii "kvm") for 
> "emulate the next instruction", the codes will be executed by emulator 
> 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? 
> 
> A testcase here:
> 
> #include <stdio.h>
> #include <string.h>
>    
> #define HYPERVISOR_INFO 0x40000000
>    
> #define CPUID(idx, eax, ebx, ecx, edx)\
>     asm volatile (\
>     "ud2a; .ascii \"kvm\"; 1: cpuid" \
>     :"=b" (*ebx), "=a" (*eax),"=c" (*ecx), "=d" (*edx)\
>         :"0"(idx) );  
>    
> void main()  
> {  
> 	unsigned int eax,ebx,ecx,edx;  
> 	char string[13];  
>    
> 	CPUID(HYPERVISOR_INFO, &eax, &ebx, &ecx, &edx);  
> 	*(unsigned int *)(string+0) = ebx;  
> 	*(unsigned int *)(string+4) = ecx;  
> 	*(unsigned int *)(string+8) = edx;  
>    
> 	string[12] = 0;  
> 	if (strncmp(string, "KVMKVMKVM\0\0\0",12) == 0)
> 		printf("kvm guest\n");  
> 	else  
> 		printf("bare hardware\n");  
> }
> 
> Suggested-by: Andrew Cooper <andrew.cooper3@...rix.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Andrew Cooper <andrew.cooper3@...rix.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/vmx.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> 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_shadow_vmcs, bool, S_IRUGO);
>  static bool __read_mostly nested = 0;
>  module_param(nested, bool, S_IRUGO);
>  
> +static bool __read_mostly fep = 0;
> +module_param(fep, bool, S_IRUGO);
> +
>  static u64 __read_mostly host_xss;
>  
>  static bool __read_mostly enable_pml = 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 = EMULTYPE_TRAP_UD;
> +
> +	if (fep) {
> +		char sig[5]; /* ud2; .ascii "kvm" */
> +		struct x86_exception e;

Don't you want to do = { };
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)) == 0) {
> +			emulation_type = 0;
> +			kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
> +		}
> +	}
>  
> -	er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
> +	er = emulate_instruction(vcpu, emulation_type);
>  	if (er == EMULATE_USER_EXIT)
>  		return 0;
>  	if (er != EMULATE_DONE)
> -- 
> 2.7.4
>