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=-0.5 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 3C4E5C47254 for ; Wed, 6 May 2020 01:38:28 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id DC0CD20735 for ; Wed, 6 May 2020 01:38:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=ellerman.id.au header.i=@ellerman.id.au header.b="GTHAsXCU" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DC0CD20735 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ellerman.id.au Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from bilbo.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 49Gzkn382wzDqmf for ; Wed, 6 May 2020 11:38:25 +1000 (AEST) Received: from ozlabs.org (bilbo.ozlabs.org [203.11.71.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 49Gzgj1MNbzDqVp for ; Wed, 6 May 2020 11:35:45 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=ellerman.id.au Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; unprotected) header.d=ellerman.id.au header.i=@ellerman.id.au header.a=rsa-sha256 header.s=201909 header.b=GTHAsXCU; dkim-atps=neutral Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mail.ozlabs.org (Postfix) with ESMTPSA id 49Gzgg6gf5z9sSr; Wed, 6 May 2020 11:35:43 +1000 (AEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ellerman.id.au; s=201909; t=1588728944; bh=KUYO+PR8+oJRVW9bjsUzpPhTlm5eKF6/oKJKp8Jeo0g=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=GTHAsXCUiXz5gZz3YepcK5hmp0cRbu6B1DOYuP4H0C577S3a6m8KBhhKu1Gpq/cJn DeWjzEeke2mr2CutozGTmoG98bACusrP5pMOowZTpqCxSD06ganBizqL1J2HHKl7rZ 0OOAfZyx5T+dwBG32Qar+O6gH/1QJ6EXezRf1wCsYbBIIsI5qaAhvKG+b9eXcgjrjs RkFBVRebIORbX84qfdQ4Cl3bgJXhiDSuj9XhZW9sIuwCxvWvrFZeGq2PUzqdCjzlLw KvjIuVsYvopt8AEH+SWbBipwDB9zPkLnWsQ9htr8IAf7zVJ4QL/q7Vy0bdPoCI0rrW h7Y2ZZ7I8E8Sw== From: Michael Ellerman To: Segher Boessenkool , Christophe Leroy Subject: Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto' In-Reply-To: <20200505155944.GO31009@gate.crashing.org> References: <23e680624680a9a5405f4b88740d2596d4b17c26.1587143308.git.christophe.leroy@c-s.fr> <87sggecv81.fsf@mpe.ellerman.id.au> <1c6379b2-7e0a-91fe-34f0-51f5adca7929@csgroup.eu> <20200505155944.GO31009@gate.crashing.org> Date: Wed, 06 May 2020 11:36:00 +1000 Message-ID: <87lfm5dev3.fsf@mpe.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-kernel@vger.kernel.org, npiggin@gmail.com, Paul Mackerras , linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Segher Boessenkool writes: > On Tue, May 05, 2020 at 05:40:21PM +0200, Christophe Leroy wrote: >> >>+#define __put_user_asm_goto(x, addr, label, op) \ >> >>+ asm volatile goto( \ >> >>+ "1: " op "%U1%X1 %0,%1 # put_user\n" \ >> >>+ EX_TABLE(1b, %l2) \ >> >>+ : \ >> >>+ : "r" (x), "m<>" (*addr) \ >> > >> >The "m<>" here is breaking GCC 4.6.3, which we allegedly still support. >> > >> >Plain "m" works, how much does the "<>" affect code gen in practice? >> > >> >A quick diff here shows no difference from removing "<>". >> >> It was recommended by Segher, there has been some discussion about it on >> v1 of this patch, see >> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/4fdc2aba6f5e51887d1cd0fee94be0989eada2cd.1586942312.git.christophe.leroy@c-s.fr/ >> >> As far as I understood that's mandatory on recent gcc to get the >> pre-update form of the instruction. With older versions "m" was doing >> the same, but not anymore. > > Yes. How much that matters depends on the asm. On older CPUs (6xx/7xx, > say) the update form was just as fast as the non-update form. On newer > or bigger CPUs it is usually executed just the same as an add followed > by the memory access, so it just saves a bit of code size. The update-forms are stdux, sthux etc. right? I don't see any change in the number of those with or without the constraint. That's using GCC 9.3.0. >> Should we ifdef the "m<>" or "m" based on GCC >> version ? > > That will be a lot of churn. Just make 4.8 minimum? As I said in my other mail that's not really up to us. We could mandate a higher minimum for powerpc, but I'd rather not. I think for now I'm inclined to just drop the "<>", and we can revisit in a release or two when hopefully GCC 4.8 has become the minimum. cheers