From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH 2/6] KVM: x86: Wrong emulation on 'xadd X, X' Date: Tue, 17 Jun 2014 07:16:56 +0200 Message-ID: <539FCF48.5060705@redhat.com> References: <1402837982-24959-1-git-send-email-namit@cs.technion.ac.il> <1402837982-24959-3-git-send-email-namit@cs.technion.ac.il> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: gleb@kernel.org, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Bandan Das , Nadav Amit Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org Il 16/06/2014 19:38, Bandan Das ha scritto: > Nadav Amit writes: > >> 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 >> --- >> 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 f0b0a10..3c8d867 100644 >> --- a/arch/x86/kvm/emulate.c >> +++ b/arch/x86/kvm/emulate.c >> @@ -4711,17 +4711,17 @@ 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); > While we are here, I think we should replace this BUG_ON with a warning > and return X86EMUL_UNHANDLEABLE if the condition is true. Sure, please post a patch and I'll apply it right away. Paolo