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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4993EC433F5 for ; Mon, 13 Dec 2021 22:05:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243381AbhLMWFH (ORCPT ); Mon, 13 Dec 2021 17:05:07 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34562 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243361AbhLMWFG (ORCPT ); Mon, 13 Dec 2021 17:05:06 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E0002C061574 for ; Mon, 13 Dec 2021 14:05:05 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 72B356126E for ; Mon, 13 Dec 2021 22:05:05 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 39756C34600; Mon, 13 Dec 2021 22:05:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1639433104; bh=Qf6Ek4AOVKGXkCaS8N754FOQ4GZ8VNQHP64V66crnEQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=tZCzX39AH4q4ioXr/G3bKEcNN56zLDou2/2aV8RcQCyTQ1u/pd7+nymSI/ZHf/EX9 Cz9yANRak/h0I32gKptVTMHGNMS2WLpMRiepFg9yJ0dqs9UHf9YR3iMRJElvonzfA4 LQMX9/lXLIiUf+jp5BIfmrYVL8IyznK6uXOalXiEO63nUU2ut8wr4IHHaHYS/jfwhX MuhRME8kiEtNCPqj1fi9Jg3ylNAnRsVKAUYfUp5izF0H2VO4RUg18tAtaGlaVNcbI+ 7lRY4+KakBolGtoXdg2CCLarXf55bx7ORqpKMEd1WxUESVfQfGGnZTypJlABuxOyGC vux/qapmO4r+A== Date: Mon, 13 Dec 2021 23:05:01 +0100 From: Frederic Weisbecker To: Mark Rutland , Peter Zijlstra Cc: linux-arm-kernel@lists.infradead.org, ardb@kernel.org, catalin.marinas@arm.com, juri.lelli@redhat.com, linux-kernel@vger.kernel.org, mingo@redhat.com, will@kernel.org Subject: Re: [PATCH 5/6] sched/preempt: add PREEMPT_DYNAMIC using static keys Message-ID: <20211213220501.GB786870@lothringen> References: <20211109172408.49641-1-mark.rutland@arm.com> <20211109172408.49641-6-mark.rutland@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211109172408.49641-6-mark.rutland@arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 09, 2021 at 05:24:07PM +0000, Mark Rutland wrote: > Where an architecture selects HAVE_STATIC_CALL but not > HAVE_STATIC_CALL_INLINE, each static call has an out-of-line trampoline > which will either branch to a callee or return to the caller. > > On such architectures, a number of constraints can conspire to make > those trampolines more complicated and potentially less useful than we'd > like. For example: > > * Hardware and software control flow integrity schemes can require the > additition of "landing pad" instructions (e.g. `BTI` for arm64), which > will also be present at the "real" callee. > > * Limited branch ranges can require that trampolines generate or load an > address into a registter and perform an indirect brach (or at least > have a slow path that does so). This loses some of the benefits of > having a direct branch. > > * Interaction with SW CFI schemes can be complicated and fragile, e.g. > requiring that we can recognise idiomatic codegen and remove > indirections understand, at least until clang proves more helpful > mechanisms for dealing with this. > > For PREEMPT_DYNAMIC, we don't need the full power of static calls, as we > really only need to enable/disable specific preemption functions. We can > achieve the same effect without a number of the pain points above by > using static keys to fold early return cases into the preemption > functions themselves rather than in an out-of-line trampoline, > effectively inlining the trampoline into the start of the function. > > For arm64, this results in good code generation, e.g. the > dynamic_cond_resched() wrapper looks as follows (with the first `B` being > replaced with a `NOP` when the function is disabled): > > | : > | bti c > | b > | mov w0, #0x0 // #0 > | ret > | mrs x0, sp_el0 > | ldr x0, [x0, #8] > | cbnz x0, > | paciasp > | stp x29, x30, [sp, #-16]! > | mov x29, sp > | bl > | mov w0, #0x1 // #1 > | ldp x29, x30, [sp], #16 > | autiasp > | ret > > ... compared to the regular form of the function: > > | <__cond_resched>: > | bti c > | mrs x0, sp_el0 > | ldr x1, [x0, #8] > | cbz x1, <__cond_resched+0x18> > | mov w0, #0x0 // #0 > | ret > | paciasp > | stp x29, x30, [sp, #-16]! > | mov x29, sp > | bl > | mov w0, #0x1 // #1 > | ldp x29, x30, [sp], #16 > | autiasp > | ret > > Any architecture which implements static keys should be able to use this > to implement PREEMPT_DYNAMIC with similar cost to non-inlined static > calls. > > Signed-off-by: Mark Rutland > Cc: Ard Biesheuvel > Cc: Frederic Weisbecker > Cc: Ingo Molnar > Cc: Juri Lelli > Cc: Peter Zijlstra Anyone has an opinion on that? Can we do better on the arm64 static call side or should we resign ourself to using that static keys direction? Also I assume that, sooner or later, arm64 will eventually need a static call implementation.... Thanks.