From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by sourceware.org (Postfix) with ESMTPS id 9E3FA3851C3A for ; Fri, 21 May 2021 16:31:52 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 9E3FA3851C3A Received: by mail.kernel.org (Postfix) with ESMTPSA id 4C343613AD; Fri, 21 May 2021 16:31:50 +0000 (UTC) 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 X-Spam-Status: No, score=-3.7 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 21 May 2021 16:31:54 -0000 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