From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 12B39C47078 for ; Fri, 21 May 2021 16:31:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D804D611ED for ; Fri, 21 May 2021 16:31:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235024AbhEUQdP (ORCPT ); Fri, 21 May 2021 12:33:15 -0400 Received: from mail.kernel.org ([198.145.29.99]:37038 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230119AbhEUQdO (ORCPT ); Fri, 21 May 2021 12:33:14 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 4C343613AD; Fri, 21 May 2021 16:31:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1621614711; bh=oxjGhdUOBodTprFXEf43FPxyTig36c7rbVQyBkMzWLg=; h=In-Reply-To:References:Date:From:To:Cc:Subject:From; b=jzlbG+n+3RHUHPR9mV9Jc108G578I6y623BdePDmtYmpwSHVZGbcGoiY+QtvobDlu W0CGj2p1PYCX29TKhS3xIIwXVtkO6kqgazbIfoHzUDKLnJ6v9cIMTlLJDNl7OL29XH d9ZKECx3M6AsYMc4g5/yViCPxLHKK3OOcwWDjLFqDFDRBC5ijwWy/Cdz1Ll/ZuM1lB b/YkisaLXur2Po7/imHwgzyQeV7j0noQioozwVWexlPwJsix838QROriKeLBsj/rvs 6zSPQK6Afqa+qqybmJSghHKct9CH1zvAQ5maz4vdAzcVldR34b3wzwkW3HWIbSDN+X 2PLIGLiGJD+JQ== Received: from compute2.internal (compute2.nyi.internal [10.202.2.42]) by mailauth.nyi.internal (Postfix) with ESMTP id 417E527C0054; Fri, 21 May 2021 12:31:49 -0400 (EDT) Received: from imap21 ([10.202.2.71]) by compute2.internal (MEProxy); Fri, 21 May 2021 12:31:49 -0400 X-ME-Sender: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeduledrvdejfedguddtvdcutefuodetggdotefrod ftvfcurfhrohhfihhlvgemucfhrghsthforghilhdpqfgfvfdpuffrtefokffrpgfnqfgh necuuegrihhlohhuthemuceftddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmd enucfjughrpefofgggkfgjfhffhffvufgtgfesthhqredtreerjeenucfhrhhomhepfdet nhguhicunfhuthhomhhirhhskhhifdcuoehluhhtoheskhgvrhhnvghlrdhorhhgqeenuc ggtffrrghtthgvrhhnpedvleehjeejvefhuddtgeegffdtjedtffegveethedvgfejieev ieeufeevuedvteenucevlhhushhtvghrufhiiigvpedtnecurfgrrhgrmhepmhgrihhlfh hrohhmpegrnhguhidomhgvshhmthhprghuthhhphgvrhhsohhnrghlihhthidqudduiedu keehieefvddqvdeifeduieeitdekqdhluhhtoheppehkvghrnhgvlhdrohhrgheslhhinh hugidrlhhuthhordhush X-ME-Proxy: Received: by mailuser.nyi.internal (Postfix, from userid 501) id E34EF51C0060; Fri, 21 May 2021 12:31:46 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface User-Agent: Cyrus-JMAP/3.5.0-alpha0-448-gae190416c7-fm-20210505.004-gae190416 Mime-Version: 1.0 Message-Id: <263a58a9-26d5-4e55-b3e1-3718baf1b81d@www.fastmail.com> In-Reply-To: <87r1i06ow2.fsf@oldenburg.str.redhat.com> References: <20210415044258.GA6318@zn.tnic> <20210415052938.GA2325@1wt.eu> <20210415054713.GB6318@zn.tnic> <20210419141454.GE9093@zn.tnic> <20210419191539.GH9093@zn.tnic> <20210419215809.GJ9093@zn.tnic> <874kf11yoz.ffs@nanos.tec.linutronix.de> <87k0ntazyn.ffs@nanos.tec.linutronix.de> <37833625-3e6b-5d93-cc4d-26164d06a0c6@intel.com> <9c8138eb-3956-e897-ed4e-426bf6663c11@intel.com> <87pmxk87th.fsf@oldenburg.str.redhat.com> <939ec057-3851-d8fb-7b45-993fa07c4cb5@intel.com> <87r1i06ow2.fsf@oldenburg.str.redhat.com> Date: Fri, 21 May 2021 09:31:26 -0700 From: "Andy Lutomirski" To: "Florian Weimer" , "Dave Hansen" Cc: "Dave Hansen via Libc-alpha" , "Len Brown" , "Rich Felker" , "Linux API" , "Bae, Chang Seok" , "the arch/x86 maintainers" , "Linux Kernel Mailing List" , "Kyle Huey" , "Borislav Petkov" , "Thomas Gleixner" , "Keno Fischer" , "Arjan van de Ven" , "Willy Tarreau" Subject: =?UTF-8?Q?Re:_Candidate_Linux_ABI_for_Intel_AMX_and_hypothetical_new_rel?= =?UTF-8?Q?ated_features?= Content-Type: text/plain;charset=utf-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Fri, May 21, 2021, at 9:19 AM, Florian Weimer wrote: > * Dave Hansen: >=20 > > On 5/21/21 7:44 AM, Florian Weimer wrote: > >> * Dave Hansen via Libc-alpha: > >>> Our system calls are *REALLY* fast. We can even do a vsyscall for= this > >>> if we want to get the overhead down near zero. Userspace can also= cache > >>> the "I did the prctl()" state in thread-local storage if it wants = to > >>> avoid the syscall. > >> Why can't userspace look at XCR0 to make the decision? > > > > The thing we're trying to avoid is a #NM exception from XFD (the new= > > first-use detection feature) that occurs on the first use of AMX. > > XCR0 will have XCR0[AMX]=3D1, even if XFD is "armed" and ready to > > generate the #NM. >=20 > I see. So essentially the hardware wants to offer transparent > initialize-on-use, but Linux does not seem to want to implement it thi= s > way. >=20 > Is there still a chance to bring the hardware and Linux into alignment= ? arch_prctl(SET_XSTATE_INIT_ON_FIRST_USE, TILE_STUFF);? As long as this is allowed to fail, I don=E2=80=99t have a huge problem = with it. I think several things here are regrettable: 1. Legacy XSTATE code might assume that XCR0 is a constant. 2. Intel virt really doesn=E2=80=99t like us context switching XCR0, alt= hough we might say that this is Intel=E2=80=99s fault and therefore Inte= l=E2=80=99s problem. AMD hardware doesn=E2=80=99t appear to have this is= sue. 3. AMX bring tangled up in XSTATE is unfortunate. The whole XSTATE mech= anism is less than amazing. IMO the best we can make of this whole situation is to make XCR0 dynamic= , but the legacy compatibility issues are potentially problematic. >=20 > Thanks, > Florian >=20 >=20