kvm/x86: fix icebp instruction handling

The undocumented 'icebp' instruction (aka 'int1') works pretty much like
'int3' in the absense of in-circuit probing equipment (except,
obviously, that it raises #DB instead of raising #BP), and is used by
some validation test-suites as such.

But Andy Lutomirski noticed that his test suite acted differently in kvm
than on bare hardware.

The reason is that kvm used an inexact test for the icebp instruction:
it just assumed that an all-zero VM exit qualification value meant that
the VM exit was due to icebp.

That is not unlike the guess that do_debug() does for the actual
exception handling case, but it's purely a heuristic, not an absolute
rule.  do_debug() does it because it wants to ascribe _some_ reasons to
the #DB that happened, and an empty %dr6 value means that 'icebp' is the
most likely casue and we have no better information.

But kvm can just do it right, because unlike the do_debug() case, kvm
actually sees the real reason for the #DB in the VM-exit interruption
information field.

So instead of relying on an inexact heuristic, just use the actual VM
exit information that says "it was 'icebp'".

Right now the 'icebp' instruction isn't technically documented by Intel,
but that will hopefully change.  The special "privileged software
exception" information _is_ actually mentioned in the Intel SDM, even
though the cause of it isn't enumerated.
Reported-by: Andy Lutomirski <luto@kernel.org>
Tested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/vmx.h

@@ -352,6 +352,7 @@ enum vmcs_field {
 #define INTR_TYPE_NMI_INTR		(2 << 8) /* NMI */
 #define INTR_TYPE_HARD_EXCEPTION	(3 << 8) /* processor exception */
 #define INTR_TYPE_SOFT_INTR             (4 << 8) /* software interrupt */
+#define INTR_TYPE_PRIV_SW_EXCEPTION	(5 << 8) /* ICE breakpoint - undocumented */
 #define INTR_TYPE_SOFT_EXCEPTION	(6 << 8) /* software exception */
 
 /* GUEST_INTERRUPTIBILITY_INFO flags. */

arch/x86/kvm/vmx.c

@@ -1045,6 +1045,13 @@ static inline bool is_machine_check(u32 intr_info)
 		(INTR_TYPE_HARD_EXCEPTION | MC_VECTOR | INTR_INFO_VALID_MASK);
 }
 
+/* Undocumented: icebp/int1 */
+static inline bool is_icebp(u32 intr_info)
+{
+	return (intr_info & (INTR_INFO_INTR_TYPE_MASK | INTR_INFO_VALID_MASK))
+		== (INTR_TYPE_PRIV_SW_EXCEPTION | INTR_INFO_VALID_MASK);
+}
+
 static inline bool cpu_has_vmx_msr_bitmap(void)
 {
 	return vmcs_config.cpu_based_exec_ctrl & CPU_BASED_USE_MSR_BITMAPS;
@@ -6179,7 +6186,7 @@ static int handle_exception(struct kvm_vcpu *vcpu)
 		      (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))) {
 			vcpu->arch.dr6 &= ~15;
 			vcpu->arch.dr6 |= dr6 | DR6_RTM;
-			if (!(dr6 & ~DR6_RESERVED)) /* icebp */
+			if (is_icebp(intr_info))
 				skip_emulated_instruction(vcpu);
 
 			kvm_queue_exception(vcpu, DB_VECTOR);