From ee212297cd425620867d4398d55d068c4203768c Mon Sep 17 00:00:00 2001 From: Nadav Amit Date: Sun, 15 Jun 2014 16:12:58 +0300 Subject: [PATCH] KVM: x86: Wrong emulation on 'xadd X, X' The emulator does not emulate the xadd instruction correctly if the two operands are the same. In this (unlikely) situation the result should be the sum of X and X (2X) when it is currently X. The solution is to first perform writeback to the source, before writing to the destination. The only instruction which should be affected is xadd, as the other instructions that perform writeback to the source use the extended accumlator (e.g., RAX:RDX). Signed-off-by: Nadav Amit Signed-off-by: Paolo Bonzini --- arch/x86/kvm/emulate.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index 439a3286406a..0d5ecbd45882 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -4736,17 +4736,17 @@ special_insn: goto done; writeback: - if (!(ctxt->d & NoWrite)) { - rc = writeback(ctxt, &ctxt->dst); - if (rc != X86EMUL_CONTINUE) - goto done; - } if (ctxt->d & SrcWrite) { BUG_ON(ctxt->src.type == OP_MEM || ctxt->src.type == OP_MEM_STR); rc = writeback(ctxt, &ctxt->src); if (rc != X86EMUL_CONTINUE) goto done; } + if (!(ctxt->d & NoWrite)) { + rc = writeback(ctxt, &ctxt->dst); + if (rc != X86EMUL_CONTINUE) + goto done; + } /* * restore dst type in case the decoding will be reused -- 2.20.1