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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43fe4cb135asm63482119f8f.6.2026.04.24.02.02.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Apr 2026 02:02:26 -0700 (PDT) Date: Fri, 24 Apr 2026 10:02:24 +0100 From: David Laight To: Peter Zijlstra Cc: Josh Poimboeuf , x86@kernel.org, linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, Joe Lawrence , Song Liu , Miroslav Benes , Petr Mladek Subject: Re: [PATCH 45/48] x86/Kconfig: Enable CONFIG_PREFIX_SYMBOLS for FineIBT Message-ID: <20260424100224.69723776@pumpkin> In-Reply-To: <20260423162902.GC641209@noisy.programming.kicks-ass.net> References: <70107aab81b01f8a2360f052ff550a9e97c30f79.1776916871.git.jpoimboe@kernel.org> <20260423084758.GY3126523@noisy.programming.kicks-ass.net> <20260423151925.GG1064669@noisy.programming.kicks-ass.net> <20260423162902.GC641209@noisy.programming.kicks-ass.net> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 23 Apr 2026 18:29:02 +0200 Peter Zijlstra wrote: > On Thu, Apr 23, 2026 at 09:23:12AM -0700, Josh Poimboeuf wrote: > > > > > Sorry, how do you get 64 here? > > > > > > DEBUG_FORCE_FUNCTION_ALIGNMENT_64B=y > > > > Ok, so in that case it would be 5-byte cfi symbol and 59-byte NOP gap. > > Or a 64-byte pfx for the !CFI case. > > Just so. > Isn't there also an average of 32 NOP bytes to align the 'gap' on a 64 byte boundary? Has anyone looked at changing clang to take a parameter for the size of the gap? That would significantly reduce the overhead for small functions. David