Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754244Ab3JQJmI (ORCPT ); Thu, 17 Oct 2013 05:42:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:5022 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753583Ab3JQJmD (ORCPT ); Thu, 17 Oct 2013 05:42:03 -0400 Date: Thu, 17 Oct 2013 12:41:46 +0300 From: Gleb Natapov To: Jan Beulich Cc: Ingo Molnar , Thomas Gleixner , Linus Torvalds , KVM list , Linux Kernel Mailing List , Peter Anvin Subject: Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors Message-ID: <20131017094146.GP15657@redhat.com> References: <525E9BFF02000078000FB74E@nat28.tlf.novell.com> <525ECEA102000078000FB906@nat28.tlf.novell.com> <525ED75402000078000FB95B@nat28.tlf.novell.com> <20131017092727.GN15657@redhat.com> <525FCB0D02000078000FBBCF@nat28.tlf.novell.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <525FCB0D02000078000FBBCF@nat28.tlf.novell.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1964 Lines: 40 On Thu, Oct 17, 2013 at 10:33:33AM +0100, Jan Beulich wrote: > >>> On 17.10.13 at 11:27, Gleb Natapov wrote: > > On Wed, Oct 16, 2013 at 05:13:40PM +0100, Jan Beulich wrote: > >> > It preserves *less* state, because the upper 32 bits of rip are now > >> > corrupted. Any 64-bit application that actually looks at the FP > >> > rip/rdp fields now get the WRONG VALUES. > >> > >> But again - this isn't being done for ordinary 64-bit applications, > >> this is only happening for KVM guests. And there not being a > >> protocol for telling the caller whether a certain context hold > >> 64-bit offsets or selector/offset pairs shouldn't be a reason to > >> think of a solution to the problem. > >> > > KVM knows what mode guest vcpu is in. is_long_mode(vcpu) will tell you > > if it is in long mode or not. No need to guess it. > > So what if that 64-bit guest OS is running a 32-bit app? You can > only positively know the _current_ guest word size when the > guest is not in long mode. > KVM obviously knows the complete state of virtual CPU. It can figure the situation above by looking at CS descriptor, not need to check is_long_mode() at all. Here is how emulator does it: kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l); ctxt->eflags = kvm_get_rflags(vcpu); ctxt->mode = (!is_protmode(vcpu)) ? X86EMUL_MODE_REAL : (ctxt->eflags & X86_EFLAGS_VM) ? X86EMUL_MODE_VM86 : cs_l ? X86EMUL_MODE_PROT64 : cs_db ? X86EMUL_MODE_PROT32 : X86EMUL_MODE_PROT16; -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/