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Wong" , Chuck Lever , Hugh Dickins , patches@lists.linux.dev, Linux-MM , mm-commits@vger.kernel.org, Mel Gorman Subject: Re: [PATCH -final] x86/clear_user: Make it faster Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1657629152; a=rsa-sha256; cv=none; b=sDXzwSF5iHh4manXrb7ZSxQgZ2Y2aNx4o3F6rjc6S5FrS2WED912XTGohtQnd6+rw85NpO WIaRPa862x8AYG/0temad3VScWMhFkVdpS77aHNacPZB2OUIIN3+Bvs/cqhBKF4v1IVjBo i5rwckmaOSgI4W2tk9UC1wioQ9oTxPo= ARC-Authentication-Results: i=1; imf10.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=LyQMDbQI; spf=pass (imf10.hostedemail.com: domain of adobriyan@gmail.com designates 209.85.221.53 as permitted sender) smtp.mailfrom=adobriyan@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1657629152; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=TzUu9/4eWAwyfJe5s1t0yONoD27sPS3zCjo2PMaad0c=; b=UrxBQ0CsOAeg0ndZYs6/RBqJA8uaKH1fXBFrQsiYmd+/kjZSy5k0KTzUUquIBJ1o0Camui 5AFDJTPJePqMun7pRKVtZHkaQka9RwNsE8DuyHoNei+VdRCdR5zt2Ssk1KWJyKDteGp+7i FxdkEaXFToYX45GyqcczEvkMuZUj+NM= X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: C2653C005C X-Rspam-User: X-Stat-Signature: tjarh1tna4bsugu67qkhggd1uegsdn4u Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=LyQMDbQI; spf=pass (imf10.hostedemail.com: domain of adobriyan@gmail.com designates 209.85.221.53 as permitted sender) smtp.mailfrom=adobriyan@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-HE-Tag: 1657629152-325336 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Jul 11, 2022 at 12:33:20PM +0200, Borislav Petkov wrote: > On Wed, Jul 06, 2022 at 12:24:12PM +0300, Alexey Dobriyan wrote: > > On Tue, Jul 05, 2022 at 07:01:06PM +0200, Borislav Petkov wrote: > > > > > + asm volatile( > > > + "1:\n\t" > > > + ALTERNATIVE_3("rep stosb", > > > + "call clear_user_erms", ALT_NOT(X86_FEATURE_FSRM), > > > + "call clear_user_rep_good", ALT_NOT(X86_FEATURE_ERMS), > > > + "call clear_user_original", ALT_NOT(X86_FEATURE_REP_GOOD)) > > > + "2:\n" > > > + _ASM_EXTABLE_UA(1b, 2b) > > > + : "+&c" (size), "+&D" (addr), ASM_CALL_CONSTRAINT > > > + : "a" (0) > > > + /* rep_good clobbers %rdx */ > > > + : "rdx"); > > > > "+c" and "+D" should be enough for 1 instruction assembly? > > I'm looking at > > e0a96129db57 ("x86: use early clobbers in usercopy*.c") > > which introduced the early clobbers and I'm thinking we want them > because "this operand is an earlyclobber operand, which is written > before the instruction is finished using the input operands" and we have > exception handling. > > But maybe you need to be more verbose as to what you mean exactly... This is the original code: -#define __do_strncpy_from_user(dst,src,count,res) \ -do { \ - long __d0, __d1, __d2; \ - might_fault(); \ - __asm__ __volatile__( \ - " testq %1,%1\n" \ - " jz 2f\n" \ - "0: lodsb\n" \ - " stosb\n" \ - " testb %%al,%%al\n" \ - " jz 1f\n" \ - " decq %1\n" \ - " jnz 0b\n" \ - "1: subq %1,%0\n" \ - "2:\n" \ - ".section .fixup,\"ax\"\n" \ - "3: movq %5,%0\n" \ - " jmp 2b\n" \ - ".previous\n" \ - _ASM_EXTABLE(0b,3b) \ - : "=&r"(res), "=&c"(count), "=&a" (__d0), "=&S" (__d1), \ - "=&D" (__d2) \ - : "i"(-EFAULT), "0"(count), "1"(count), "3"(src), "4"(dst) \ - : "memory"); \ -} while (0) I meant to say that earlyclobber is necessary only because the asm body is more than 1 instruction so there is possibility of writing to some outputs before all inputs are consumed. If asm body is 1 insn there is no such possibility at all. Now "rep stosb" is 1 instruction and two alterantive functions masquarade as single instruction.