From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) by sourceware.org (Postfix) with ESMTPS id 76B6A383303D for ; Fri, 7 May 2021 19:22:20 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 76B6A383303D From: Thomas Gleixner 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 X-Spam-Status: No, score=-2.2 required=5.0 tests=BAYES_00, 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, 07 May 2021 19:22:21 -0000 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