x86 emulator: emulate CPUID instruction

This patch emulates CPUID instruction. It should work but when I'm testing it in kvm-userspace, the cpuid instruction seems to have no effect (I mean eax == ebx == ecx == edx == 0x0). Should kvm_cpuid_emulate() be modified? Is cpuid instruction correctly implemented?

Regards,

---
 arch/x86/kvm/x86_emulate.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

index d174db7..4d1821c 100644
@@ -237,7 +237,7 @@ static u32 twobyte_table[256] = {
 	/* 0x90 - 0x9F */
 	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
 	/* 0xA0 - 0xA7 */
-	0, 0, 0, DstMem | SrcReg | ModRM | BitOp,
+	0, 0, ImplicitOps, DstMem | SrcReg | ModRM | BitOp,
 	DstMem | SrcReg | Src2ImmByte | ModRM,
 	DstMem | SrcReg | Src2CL | ModRM, 0, 0,
 	/* 0xA8 - 0xAF */

@@ -2035,6 +2035,9 @@ twobyte_insn:
 		c->dst.type = OP_NONE;
 		break;
 	}
+	case 0xa2:		/* cpuid */
+		kvm_emulate_cpuid(ctxt->vcpu);
+		break;
 	case 0xa3:
 	      bt:		/* bt */
 		c->dst.type = OP_NONE;

1.6.0.4.623.g171d7

It doesn't have any effect because writeback() writes back the cached registers back into vcpu->arch.regs, overwriting kvm_cpuid_emulate()'s effect.

You need to first modify kvm_cpuid_emulate() to accept the registers as in/out parameters, and pass the emulator's registers which can be later written back.

An alternative implementation can modify the emulator not to work on a temporarary copy, instead something like this:

decode():
copy registers to backup store

execute():
start executing
if fail:
copy registers from backup store to main state

The later approach is more invasive, but has the advantage of speeding up the emulator (since usually emulation will succeed, so we don't need to writeback)

error compiling committee.c: too many arguments to function