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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 7AFA3C433B4 for ; Fri, 7 May 2021 19:22:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3D81161003 for ; Fri, 7 May 2021 19:22:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229864AbhEGTXV (ORCPT ); Fri, 7 May 2021 15:23:21 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:51386 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229606AbhEGTXU (ORCPT ); Fri, 7 May 2021 15:23:20 -0400 From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1620415339; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ORUuoUk8DJpvMhKQmGSDce5dSlAguNz2DeA4Kqt7HbQ=; b=bjhQDjeBlOjjsz24EuAiTVQam0CwX43eBcx/62NQHMC/Gb5w0WUzKL9Udvo20IvjRLQeoV GeumJrz+67KiP6l60XJiDJq0BFWsn8qS0rFknBhS6SbOHYk6qX1tj3X6gc8kvSEbrdx5p0 7OZpcEiZhtsn5KA9Y0zOAN9dWrnIGvTA/voSCJdcXK5RgMhWCdZf5oe1322Oe8ULw+RF1V /RQss4faY0+057dfPvfMP8uXrMb9hSC8igfriVyw27yKci7iXHk7ZdRIKWcclpOA5faVWE 208MneA3Q5/7KgtJiLzOAz+e5GWyoM0tiFVJ/k18y4guswq9zgGnqStvTUNuEA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1620415339; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ORUuoUk8DJpvMhKQmGSDce5dSlAguNz2DeA4Kqt7HbQ=; b=tclfaHK/1ZJRJghVXP1ksCNWC85cpf/DqYc+NVH3+t2rPEjNLEtjYGBffvj0+MxFoT6CLy H/sAnp8YPNvZiRDA== To: Andy Lutomirski Cc: Dave Hansen , Florian Weimer , Len Brown , Borislav Petkov , Willy Tarreau , Andy Lutomirski , "Bae\, Chang Seok" , X86 ML , LKML , linux-abi@vger.kernel.org, "libc-alpha\@sourceware.org" , Rich Felker , Kyle Huey , Keno Fischer Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related features In-Reply-To: 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> <87bl9s8kfb.fsf@oldenburg.str.redhat.com> <5d3d513b-77d6-e2e2-779e-ff3ea33deba3@intel.com> <87o8dmmljh.ffs@nanos.tec.linutronix.de> Date: Fri, 07 May 2021 21:22:18 +0200 Message-ID: <87lf8qmjrp.ffs@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 07 2021 at 11:50, Andy Lutomirski wrote: > On Fri, May 7, 2021 at 11:44 AM Thomas Gleixner wrote: >> >> On Mon, May 03 2021 at 06:43, Dave Hansen wrote: >> > On 5/2/21 10:18 PM, Florian Weimer wrote: >> >>> 5. If the feature is enabled in XCR0, the user happily uses it. >> >>> >> >>> For AMX, Linux implements "transparent first use" >> >>> so that it doesn't have to allocate 8KB context switch >> >>> buffers for tasks that don't actually use AMX. >> >>> It does this by arming XFD for all tasks, and taking a #NM >> >>> to allocate a context switch buffer only for those tasks >> >>> that actually execute AMX instructions. >> >> What happens if the kernel cannot allocate that additional context >> >> switch buffer? >> > >> > Well, it's vmalloc()'d and currently smaller that the kernel stack, >> > which is also vmalloc()'d. While it can theoretically fail, if it >> > happens you have bigger problems on your hands. >> >> Such a buffer allocation might also exceed a per process/cgroup >> limitation. Anything else which is accounted happens in syscall context >> which then returns an error on which the application can react. >> >> So what's the consequence when the allocation fails? Kill it right away >> from #NM? Kill it on the first signal? Do nothing and see what happens? >> > It has to be an immediate signal or kill. Killing it right there is the only sensible thing to do. > A failure to load FPU state is somewhat tolerable (and has to be for > CET), but a failure to *save* FPU state on a context switch would be a > really nasty can of worms. :) > At the very least we will want arch_prctl(ARCH_ALLOCTE_XSTATE, mask) > to allow HPC workloads to manually allocate the state and get an error > code if it fails. Yes. Thanks, tglx