Received: by 2002:ac0:a5b6:0:0:0:0:0 with SMTP id m51-v6csp1247283imm; Fri, 15 Jun 2018 13:44:44 -0700 (PDT) X-Google-Smtp-Source: ADUXVKIw0w9KQ1DaN5f1QBPynYew49h69g7Qby0vBQ1+fiN5PvVDvmSomHN9+9y0BxeYKuyP4XW1 X-Received: by 2002:a62:a21b:: with SMTP id m27-v6mr3541904pff.248.1529095484052; Fri, 15 Jun 2018 13:44:44 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1529095484; cv=none; d=google.com; s=arc-20160816; b=zMO7RcJ7LICCRVQl6zKaypkqaZkqxYDce+SL1WXWZcBFSFwtb3gmHzXKrjAtw1KgFg WyIa+o8/hMKBf46RTuV/uhmn0LqemlsWs8/RzgbQqr25sMESqD0/MSz9lQk4H51wYbIh /rpmwl4vUmX7dY2KQaLonHu4lCQNZEFBpnBqPDd12c9eL9oJEzCFeghKWvvzvjpXgbhQ OWP6UATkf75VvC3KSAbK1BQIOUcwBBJkcBOPA5eprOfjinWTH5HDw2VrIQ3SRSUS1QKy GhLA1umDg0HVsq+3PMkBpL5Jyq6ffV1TtiPNe0n7VZsrbw3BI04c8omUflyXDJa+MCAW yq5A== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:content-transfer-encoding :content-language:in-reply-to:mime-version:user-agent:date :message-id:autocrypt:openpgp:from:cc:references:to:subject :arc-authentication-results; bh=ofiajijy1hO6P8Gs+J330lSc+fzVPL3TNrwFBK5NE44=; b=S4EPp6/8kiQfF6qeklLesqfIEmrW/Ucfn8pDPMcvopn7Yy93o0q+asFCdpNmUBTYtQ Wko6CzQj0NaMGQTnF6Y5wpIMGBfuIlFq6tn5s1bwtaufrH6Wj+tRtg9rzlWBJSTKJMuv C5kX/RtTCp15UMxsvXO/5/2ZAYHsN4oDo4R1J5m8fmlU0zkTlXo2p+8lxsq9I96c+sVJ KgJqUf0uFWQu+/heLO0wLfWkQprs0XaNlAQNDut5yzA7kajjdtL5NcyIrSQHhMSxS94+ 9CPJHOvwhIrljA0DqqTILratJWyihUHhu3fDt4tp7Acuc3xEhRHzHMQdsy7zaduihhWc qC1Q== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=intel.com Return-Path: Received: from vger.kernel.org (vger.kernel.org. [209.132.180.67]) by mx.google.com with ESMTP id k82-v6si8797335pfa.53.2018.06.15.13.44.29; Fri, 15 Jun 2018 13:44:44 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=intel.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756404AbeFOUoH (ORCPT + 99 others); Fri, 15 Jun 2018 16:44:07 -0400 Received: from mga14.intel.com ([192.55.52.115]:29796 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752815AbeFOUoG (ORCPT ); Fri, 15 Jun 2018 16:44:06 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga001.jf.intel.com ([10.7.209.18]) by fmsmga103.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 15 Jun 2018 13:44:03 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.51,228,1526367600"; d="scan'208";a="64951203" Received: from ray.jf.intel.com (HELO [10.7.201.139]) ([10.7.201.139]) by orsmga001.jf.intel.com with ESMTP; 15 Jun 2018 13:44:03 -0700 Subject: Re: Lazy FPU restoration / moving kernel_fpu_end() to context switch To: "Jason A. Donenfeld" , Andrew Lutomirski References: Cc: riel@surriel.com, LKML , X86 ML From: Dave Hansen Openpgp: preference=signencrypt Autocrypt: addr=dave.hansen@linux.intel.com; keydata= xsFNBE6HMP0BEADIMA3XYkQfF3dwHlj58Yjsc4E5y5G67cfbt8dvaUq2fx1lR0K9h1bOI6fC oAiUXvGAOxPDsB/P6UEOISPpLl5IuYsSwAeZGkdQ5g6m1xq7AlDJQZddhr/1DC/nMVa/2BoY 2UnKuZuSBu7lgOE193+7Uks3416N2hTkyKUSNkduyoZ9F5twiBhxPJwPtn/wnch6n5RsoXsb ygOEDxLEsSk/7eyFycjE+btUtAWZtx+HseyaGfqkZK0Z9bT1lsaHecmB203xShwCPT49Blxz VOab8668QpaEOdLGhtvrVYVK7x4skyT3nGWcgDCl5/Vp3TWA4K+IofwvXzX2ON/Mj7aQwf5W iC+3nWC7q0uxKwwsddJ0Nu+dpA/UORQWa1NiAftEoSpk5+nUUi0WE+5DRm0H+TXKBWMGNCFn c6+EKg5zQaa8KqymHcOrSXNPmzJuXvDQ8uj2J8XuzCZfK4uy1+YdIr0yyEMI7mdh4KX50LO1 pmowEqDh7dLShTOif/7UtQYrzYq9cPnjU2ZW4qd5Qz2joSGTG9eCXLz5PRe5SqHxv6ljk8mb ApNuY7bOXO/A7T2j5RwXIlcmssqIjBcxsRRoIbpCwWWGjkYjzYCjgsNFL6rt4OL11OUF37wL QcTl7fbCGv53KfKPdYD5hcbguLKi/aCccJK18ZwNjFhqr4MliQARAQABzShEYXZpZCBDaHJp c3RvcGhlciBIYW5zZW4gPGRhdmVAc3I3MS5uZXQ+wsF7BBMBAgAlAhsDBgsJCAcDAgYVCAIJ CgsEFgIDAQIeAQIXgAUCTo3k0QIZAQAKCRBoNZUwcMmSsMO2D/421Xg8pimb9mPzM5N7khT0 2MCnaGssU1T59YPE25kYdx2HntwdO0JA27Wn9xx5zYijOe6B21ufrvsyv42auCO85+oFJWfE K2R/IpLle09GDx5tcEmMAHX6KSxpHmGuJmUPibHVbfep2aCh9lKaDqQR07gXXWK5/yU1Dx0r VVFRaHTasp9fZ9AmY4K9/BSA3VkQ8v3OrxNty3OdsrmTTzO91YszpdbjjEFZK53zXy6tUD2d e1i0kBBS6NLAAsqEtneplz88T/v7MpLmpY30N9gQU3QyRC50jJ7LU9RazMjUQY1WohVsR56d ORqFxS8ChhyJs7BI34vQusYHDTp6PnZHUppb9WIzjeWlC7Jc8lSBDlEWodmqQQgp5+6AfhTD kDv1a+W5+ncq+Uo63WHRiCPuyt4di4/0zo28RVcjtzlGBZtmz2EIC3vUfmoZbO/Gn6EKbYAn rzz3iU/JWV8DwQ+sZSGu0HmvYMt6t5SmqWQo/hyHtA7uF5Wxtu1lCgolSQw4t49ZuOyOnQi5 f8R3nE7lpVCSF1TT+h8kMvFPv3VG7KunyjHr3sEptYxQs4VRxqeirSuyBv1TyxT+LdTm6j4a mulOWf+YtFRAgIYyyN5YOepDEBv4LUM8Tz98lZiNMlFyRMNrsLV6Pv6SxhrMxbT6TNVS5D+6 UorTLotDZKp5+M7BTQRUY85qARAAsgMW71BIXRgxjYNCYQ3Xs8k3TfAvQRbHccky50h99TUY sqdULbsb3KhmY29raw1bgmyM0a4DGS1YKN7qazCDsdQlxIJp9t2YYdBKXVRzPCCsfWe1dK/q 66UVhRPP8EGZ4CmFYuPTxqGY+dGRInxCeap/xzbKdvmPm01Iw3YFjAE4PQ4hTMr/H76KoDbD cq62U50oKC83ca/PRRh2QqEqACvIH4BR7jueAZSPEDnzwxvVgzyeuhwqHY05QRK/wsKuhq7s UuYtmN92Fasbxbw2tbVLZfoidklikvZAmotg0dwcFTjSRGEg0Gr3p/xBzJWNavFZZ95Rj7Et db0lCt0HDSY5q4GMR+SrFbH+jzUY/ZqfGdZCBqo0cdPPp58krVgtIGR+ja2Mkva6ah94/oQN lnCOw3udS+Eb/aRcM6detZr7XOngvxsWolBrhwTQFT9D2NH6ryAuvKd6yyAFt3/e7r+HHtkU kOy27D7IpjngqP+b4EumELI/NxPgIqT69PQmo9IZaI/oRaKorYnDaZrMXViqDrFdD37XELwQ gmLoSm2VfbOYY7fap/AhPOgOYOSqg3/Nxcapv71yoBzRRxOc4FxmZ65mn+q3rEM27yRztBW9 AnCKIc66T2i92HqXCw6AgoBJRjBkI3QnEkPgohQkZdAb8o9WGVKpfmZKbYBo4pEAEQEAAcLB XwQYAQIACQUCVGPOagIbDAAKCRBoNZUwcMmSsJeCEACCh7P/aaOLKWQxcnw47p4phIVR6pVL e4IEdR7Jf7ZL00s3vKSNT+nRqdl1ugJx9Ymsp8kXKMk9GSfmZpuMQB9c6io1qZc6nW/3TtvK pNGz7KPPtaDzvKA4S5tfrWPnDr7n15AU5vsIZvgMjU42gkbemkjJwP0B1RkifIK60yQqAAlT YZ14P0dIPdIPIlfEPiAWcg5BtLQU4Wg3cNQdpWrCJ1E3m/RIlXy/2Y3YOVVohfSy+4kvvYU3 lXUdPb04UPw4VWwjcVZPg7cgR7Izion61bGHqVqURgSALt2yvHl7cr68NYoFkzbNsGsye9ft M9ozM23JSgMkRylPSXTeh5JIK9pz2+etco3AfLCKtaRVysjvpysukmWMTrx8QnI5Nn5MOlJj 1Ov4/50JY9pXzgIDVSrgy6LYSMc4vKZ3QfCY7ipLRORyalFDF3j5AGCMRENJjHPD6O7bl3Xo 4DzMID+8eucbXxKiNEbs21IqBZbbKdY1GkcEGTE7AnkA3Y6YB7I/j9mQ3hCgm5muJuhM/2Fr OPsw5tV/LmQ5GXH0JQ/TZXWygyRFyyI2FqNTx4WHqUn3yFj8rwTAU1tluRUYyeLy0ayUlKBH ybj0N71vWO936MqP6haFERzuPAIpxj2ezwu0xb1GjTk4ynna6h5GjnKgdfOWoRtoWndMZxbA z5cecg== Message-ID: <7e415c4d-415b-0ff6-5488-1681e522d2dc@linux.intel.com> Date: Fri, 15 Jun 2018 13:42:14 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/15/2018 01:33 PM, Jason A. Donenfeld wrote: > On Fri, Jun 15, 2018 at 8:32 PM Andy Lutomirski wrote: >> quite in the form you imagined. The idea that we've tossed around is >> to restore FPU state on return to user mode. Roughly, we'd introduce >> a new thread flag TIF_FPU_UNLOADED (name TBD). >> prepare_exit_to_usermode() would notice this flag, copy the fpstate to >> fpregs, and clear the flag. (Or maybe exit_to_usermode_loop() -- No >> one has quite thought it through, but I think it should be outside the >> loop.) We'd update all the FPU accessors to understand the flag. > Yes! This is exactly what I was thinking. Then those calls to begin() > and end() could be placed as close to the actual FPU usage as > possible. Andy, what was the specific concern about PKRU? That we might do: kernel_fpu_begin(); <- Saves the first time something() kernel_fpu_end(); <- Does not XRSTOR copy_from_user(); <- Sees old PKRU, does the wrong thing prepare_exit_to_usermode(); <- Does the XRSTOR // only now does PKRU have the right value SYSRET/IRET ? Does that *matter* unless something() modified PKRU? We could just make the rule that nobody is supposed to mess with it and that it's not covered by kernel_fpu_begin/end() semantics. We could even theoretically enforce that in a debug environment if we watch its value.