KVM: x86: Add an emulation flag for implicit system access

Add an emulation flag X86EMUL_F_IMPLICIT to identify implicit system access
in instruction emulation.  Don't bother wiring up any usage at this point,
as Linear Address Space Separation (LASS) will be the first "real" consumer
of the flag and LASS support will require dedicated hooks, i.e. there
aren't any existing calls where passing X86EMUL_F_IMPLICIT is meaningful.

Add the IMPLICIT flag even though there's no imminent usage so that
Linear Address Masking (LAM) support can reference the flag to document
that addresses for implicit accesses aren't untagged.

Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Tested-by: Xuelian Guo <xuelian.guo@intel.com>
Link: https://lore.kernel.org/r/20230913124227.12574-4-binbin.wu@linux.intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/kvm_emulate.h

@@ -91,6 +91,7 @@ struct x86_instruction_info {
 /* x86-specific emulation flags */
 #define X86EMUL_F_WRITE			BIT(0)
 #define X86EMUL_F_FETCH			BIT(1)
+#define X86EMUL_F_IMPLICIT		BIT(2)
 
 struct x86_emulate_ops {
 	void (*vm_bugged)(struct x86_emulate_ctxt *ctxt);