[PATCH v4 0/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

[Wanpeng Li]

There is no easy way to force KVM to run an instruction through the emulator 
(by design as that will expose the x86 emulator as a significant attack-surface).
However, we do wish to expose the x86 emulator in case we are testing it
(e.g. via kvm-unit-tests). Therefore, this patch adds a "force emulation prefix"
that is designed to raise #UD which KVM will trap and it's #UD exit-handler will
match "force emulation prefix" to run instruction after prefix by the x86 emulator.
To not expose the x86 emulator by default, we add a module parameter that should 
be off by default.

A simple testcase here:

#include <stdio.h>
#include <string.h>
   
#define HYPERVISOR_INFO 0x40000000
   
#define CPUID(idx, eax, ebx, ecx, edx) \
    asm volatile ( \
    "ud2a; .ascii \"kvm\"; 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");  
}

v3 -> v4:
 * forwarding emulation failure to userspace
v2 -> v3:
 * fix compile warning
v1 -> v2:
 * update patch descriptions
 * move handle_ud to x86.c, shared by vmx and svm
 * the parameter is in kvm module 
 * rename parameter to force_emulation_prefix

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Liran Alon <liran.alon@xxxxxxxxxx>

Wanpeng Li (2):
  KVM: X86: Introduce handle_ud()
  KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

 arch/x86/kvm/svm.c |  9 +--------
 arch/x86/kvm/vmx.c | 10 ++--------
 arch/x86/kvm/x86.c | 28 ++++++++++++++++++++++++++++
 arch/x86/kvm/x86.h |  2 ++
 4 files changed, 33 insertions(+), 16 deletions(-)

-- 
2.7.4