From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756283AbcCBAq4 (ORCPT ); Tue, 1 Mar 2016 19:46:56 -0500 Received: from mga09.intel.com ([134.134.136.24]:63982 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752794AbcCBAqm (ORCPT ); Tue, 1 Mar 2016 19:46:42 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.22,524,1449561600"; d="scan'208";a="927506903" Subject: Re: [PATCH v3 9/9] x86/xsaves: Re-enable XSAVES To: Yu-cheng Yu References: <787ba3e73f657b06c02464ae522a47905ed503b8.1456524359.git.yu-cheng.yu@intel.com> <56D62C1C.3050806@linux.intel.com> <20160302003443.GA30899@test-lenovo> Cc: x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org, Andy Lutomirski , Borislav Petkov , Sai Praneeth Prakhya , "Ravi V. Shankar" , Fenghua Yu From: Dave Hansen Message-ID: <56D637B5.3030907@linux.intel.com> Date: Tue, 1 Mar 2016 16:45:41 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 MIME-Version: 1.0 In-Reply-To: <20160302003443.GA30899@test-lenovo> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/01/2016 04:34 PM, Yu-cheng Yu wrote: > On Tue, Mar 01, 2016 at 03:56:12PM -0800, Dave Hansen wrote: >> On 02/29/2016 09:42 AM, Yu-cheng Yu wrote: >>> - setup_clear_cpu_cap(X86_FEATURE_XSAVES); >>> + if (!config_enabled(CONFIG_X86_64)) >>> + setup_clear_cpu_cap(X86_FEATURE_XSAVES); >>> } >> >> I think we need a much better explanation of this for posterity. Why >> are we not supporting this now, and what would someone have to do in the >> future in order to enable it? >> > If anyone is using this newer feature, then that user is most likely using > a 64-bit capable processor and a 64-bit kernel. The intention here is to > take out the complexity and any potential of error. If the user removes > the restriction and builds a private kernel, it should work but we have > not checked all possible combinations. I will put these in the comments. A user can go download a 32-bit version of Ubuntu or Debian and install it on a 64-bit processor today. It's a very easy mistake to make when downloading the install CD. In any case, I don't have a _problem_ with leaving i386 in the dust here. I just want us to be very explicit about what we are doing. >>> + /* >>> + * Make it clear that XSAVES supervisor states are not yet >>> + * implemented should anyone expect it to work by changing >>> + * bits in XFEATURE_MASK_* macros and XCR0. >>> + */ >>> + WARN_ONCE((xfeatures_mask & XFEATURE_MASK_SUPERVISOR), >>> + "x86/fpu: XSAVES supervisor states are not yet implemented.\n"); >>> + >>> cr4_set_bits(X86_CR4_OSXSAVE); >>> xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask); >>> } >> >> Let's also do a: >> >> xfeatures_mask &= ~XFEATURE_MASK_SUPERVISOR; >> >> Otherwise, we have a broken system at the moment. >> > Currently, if anyone sets any supervisor state in xfeatures_mask, the > kernel prints out the warning then goes into a protection fault. > That is a very strong indication to the user. Do we want to mute it? By "goes into a protection fault", do you mean that it doesn't boot? I'd just rather we put the kernel in a known-safe configuration (masking supervisor state out of xfeatures_mask) rather than rely on the general protection fault continuing to be generated by whatever is generating it.