[PATCH 4/5] KVM: x86: RSI/RDI/RCX are zero-extended when affected by string ops

[Nadav Amit <namit@cs.technion.ac.il>]

When using address-size override prefix with string instructions in long-mode,
ESI/EDI/ECX are zero extended if they are affected by the instruction
(incremented/decremented).  Currently, the KVM emulator does not do so.

In addition, although it is not well-documented, when address override prefix
is used with REP-string instruction, RCX high half is zeroed even if ECX was
zero on the first iteration. Therefore, the emulator should clear the upper
part of RCX in this case, as x86 CPUs do.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
---
:100644 100644 69e2636... a69ed67... M	arch/x86/kvm/emulate.c
 arch/x86/kvm/emulate.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 69e2636..a69ed67 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -491,6 +491,8 @@ register_address_increment(struct x86_emulate_ctxt *ctxt, unsigned long *reg, in
 	else
 		mask = ad_mask(ctxt);
 	masked_increment(reg, mask, inc);
+	if (ctxt->ad_bytes == 4)
+		*reg &= 0xffffffff;
 }
 
 static void rsp_increment(struct x86_emulate_ctxt *ctxt, int inc)
@@ -4567,6 +4569,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 	if (ctxt->rep_prefix && (ctxt->d & String)) {
 		/* All REP prefixes have the same first termination condition */
 		if (address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) == 0) {
+			if (ctxt->ad_bytes == 4)
+				*reg_write(ctxt, VCPU_REGS_RCX) = 0;
 			ctxt->eip = ctxt->_eip;
 			goto done;
 		}
-- 
1.7.10.4