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[60.240.121.136]) by smtp.gmail.com with ESMTPSA id l83-v6sm51243749pfi.172.2018.10.09.03.54.34 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 09 Oct 2018 03:54:37 -0700 (PDT) Date: Tue, 9 Oct 2018 20:54:30 +1000 From: Nicholas Piggin To: Benjamin Herrenschmidt Cc: Christophe Leroy , Paul Mackerras , Michael Ellerman , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v6 0/9] powerpc: Switch to CONFIG_THREAD_INFO_IN_TASK Message-ID: <20181009205430.351134eb@roar.ozlabs.ibm.com> In-Reply-To: References: X-Mailer: Claws Mail 3.17.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 08 Oct 2018 20:59:56 +1100 Benjamin Herrenschmidt wrote: > On Mon, 2018-10-08 at 09:16 +0000, Christophe Leroy wrote: > > The purpose of this serie is to activate CONFIG_THREAD_INFO_IN_TASK which > > moves the thread_info into task_struct. > > We need to make sure we don't have code that assumes that we don't take > faults on TI access. > > On ppc64, the stack SLB entries are bolted, which means the TI is too. > > We might have code that assumes that we don't get SLB faults when > accessing TI. If not, we're fine but that needs a close look. Oh, we do. I think the entry side might be okay, but on exit we have at least one (in syscall and interrupt exit both): /* * Disable interrupts so current_thread_info()->flags can't change, * and so that we don't get interrupted after loading SRR0/1. */ #ifdef CONFIG_PPC_BOOK3E wrteei 0 #else /* * For performance reasons we clear RI the same time that we * clear EE. We only need to clear RI just before we restore r13 * below, but batching it with EE saves us one expensive mtmsrd call. * We have to be careful to restore RI if we branch anywhere from * here (eg syscall_exit_work). */ li r11,0 mtmsrd r11,1 #endif /* CONFIG_PPC_BOOK3E */ ld r9,TI_FLAGS(r12) So taking an SLB there will cause an unrecoverable. I think we can probably get rid of that optimization for now. I've found for non-trivial syscalls it's often a loss if FP was used. I have a couple of different options I'm working on to get rid of the mtmsrd entirely we can go with instead (but I don't think those have to come before Christophe's patch). Thanks, Nick