Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752308AbbDUAfq (ORCPT ); Mon, 20 Apr 2015 20:35:46 -0400 Received: from mail.kernel.org ([198.145.29.136]:49082 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751273AbbDUAfo (ORCPT ); Mon, 20 Apr 2015 20:35:44 -0400 Message-ID: <55359B57.3070008@kernel.org> Date: Mon, 20 Apr 2015 17:35:35 -0700 From: Andy Lutomirski User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: Andrew Cooper , Xen-devel CC: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, Konrad Rzeszutek Wilk , Boris Ostrovsky , David Vrabel , Rusty Russell , lguest@lists.ozlabs.org, X86 ML , Denys Vlasenko Subject: Re: [PATCH] [RFC] x86/cpu: Fix SMAP check in PVOPS environments References: <1429549782-12962-1-git-send-email-andrew.cooper3@citrix.com> In-Reply-To: <1429549782-12962-1-git-send-email-andrew.cooper3@citrix.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2546 Lines: 57 On 04/20/2015 10:09 AM, Andrew Cooper wrote: > There appears to be no formal statement of what pv_irq_ops.save_fl() is > supposed to return precisely. Native returns the full flags, while lguest and > Xen only return the Interrupt Flag, and both have comments by the > implementations stating that only the Interrupt Flag is looked at. This may > have been true when initially implemented, but no longer is. > > To make matters worse, the Xen PVOP leaves the upper bits undefined, making > the BUG_ON() undefined behaviour. Experimentally, this now trips for 32bit PV > guests on Broadwell hardware. The BUG_ON() is consistent for an individual > build, but not consistent for all builds. It has also been a sitting timebomb > since SMAP support was introduced. > > Use native_save_fl() instead, which will obtain an accurate view of the AC > flag. > > Signed-off-by: Andrew Cooper > CC: Thomas Gleixner > CC: Ingo Molnar > CC: H. Peter Anvin > CC: x86@kernel.org > CC: linux-kernel@vger.kernel.org > CC: Konrad Rzeszutek Wilk > CC: Boris Ostrovsky > CC: David Vrabel > CC: xen-devel > CC: Rusty Russell > CC: lguest@lists.ozlabs.org > > --- > This patch is RFC because I am not certain that native_save_fl() is > necessarily the correct solution on lguest, but it does seem that setup_smap() > wants to check the actual AC bit, rather than an idealised value. > > A different approach, given the dual nature of the AC flag now is to gate > setup_smap() on a kernel rpl of 0. SMAP necessarily can't be used in a > paravirtual situation where the kernel runs in cpl > 0. > > Another different approach would be to formally state that > pv_irq_ops.save_fl() needs to return all the flags, which would make > local_irq_save() safe to use in this circumstance, but that makes a hotpath > longer for the sake of a single boot time check. ...which reminds me: Why does native_restore_fl restore anything other than IF? A branch and sti should be considerably faster than popf. Also, if we did this, could Xen use PVI and then use native_restore_fl and avoid lots of pvops? --Andy -- 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/