From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 0B718480320; Wed, 6 May 2026 14:30:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=63.228.1.57 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778077811; cv=none; b=TdJVkhM960nny5jSKqQVOiUTJpvfRGWUll+9HudJhMOhyZr/S5q4DtRG4kRMR90/l0ERd162tOHkJC3P2LFS9PZRNjPexzMZ3SDFZ076diK5tEzUCXODgb7e4csxOxEViweI+ZK+JZUNXdkfuKjP1VAkwMwurZkB3JTGHGBH/zk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778077811; c=relaxed/simple; bh=SERVxrWoC/AibqdeLUCiOyktDNd01Ca70maYmcqPMvo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=B3XutOUxzuDPrg/W27v3htw2P4U6jpcPBoYXv19IdYNvETLsQdZmBNpqUYfc6Nwb/F4H/pCW5FtdeY9VG3oZvYYv3tx8N7GGUxr+hrp4pId3qUZHhfHHsDZkjvr+Ft5rwAnXCCfsxONSjZB4uNjjPsKh947DZRepLt/6DgwmuO0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org; spf=pass smtp.mailfrom=kernel.crashing.org; arc=none smtp.client-ip=63.228.1.57 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=kernel.crashing.org Received: from gate.crashing.org (localhost [127.0.0.1]) by gate.crashing.org (8.18.1/8.18.1/Debian-2) with ESMTP id 646ESaYa352811; Wed, 6 May 2026 09:28:36 -0500 Received: (from segher@localhost) by gate.crashing.org (8.18.1/8.18.1/Submit) id 646ESUw9352806; Wed, 6 May 2026 09:28:30 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Wed, 6 May 2026 09:28:30 -0500 From: Segher Boessenkool To: Peter Zijlstra Cc: "Christophe Leroy (CS GROUP)" , Sathvika Vasireddy , nathan@kernel.org, nsc@kernel.org, maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com, jpoimboe@kernel.org, ojeda@kernel.org, masahiroy@kernel.org, lossin@kernel.org, tamird@kernel.org, thomas.weissschuh@linutronix.de, rostedt@goodmis.org, ihor.solodrai@linux.dev, thuth@redhat.com, pmladek@suse.com, aliceryhl@google.com, elver@google.com, kees@kernel.org, legion@kernel.org, ardb@kernel.org, yuxuan.zuo@outlook.com, alexghiti@rivosinc.com, alexandre.chartre@oracle.com, bp@alien8.de, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v1 1/6] objtool/powerpc: Add build-time fixup of alternate feature branch targets Message-ID: References: <20260505084628.17940-1-sv@linux.ibm.com> <20260505084628.17940-2-sv@linux.ibm.com> <20260505144539.GX3126523@noisy.programming.kicks-ass.net> <20260505144949.GO3102924@noisy.programming.kicks-ass.net> <669fbc9a-a243-43e8-8888-93bfb9d6ee12@kernel.org> <20260506071753.GA3126523@noisy.programming.kicks-ass.net> 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-Disposition: inline In-Reply-To: <20260506071753.GA3126523@noisy.programming.kicks-ass.net> On Wed, May 06, 2026 at 09:17:53AM +0200, Peter Zijlstra wrote: > On Tue, May 05, 2026 at 05:48:32PM +0200, Christophe Leroy (CS GROUP) wrote: > > bclr (which is the return INSN_RETURN) has type 19 > > > > By the way you can have a look at https://patchwork.ozlabs.org/project/linuxppc-dev/patch/bfa8364da047d8610a09458a1cd924a0566aedbb.1736955567.git.christophe.leroy@csgroup.eu/ > > That is indeed more; isn't bcl something like COND_CALL ? (another one > of them things we don't have). Yeah, all of bc[l][a] are conditional branches. "a" (AA=1) means the branch target is an absolute address (in the top or bottom 8kB of the address space), and "l" (LK=1) means set LR to the NIA ("next instruction address") before doing the branch. The BO field describes the condition. 0x14 means "always". (But there also are primary opcodes for b[l][a], with a bigger target field, 24 bits instead of the 14 bits of bc[l][a]; you do not have the BI and BO fields then of course) ("branch index" and "branch option" btw). > > That patch has all the objtool decoding. By the way objtool is missing a > > INSN_CONDITIONAL_RETURN, also see https://patchwork.ozlabs.org/project/linuxppc-dev/patch/537e5d8f181b1f1c2b8918f1aefa1dba3f972c03.1736955567.git.christophe.leroy@csgroup.eu/ > > Right, that is not something x86 has, but I don't see a reason we can't > add that. With return thunks, Clang (and I've heard GCC is also > considering this) does something very close to conditional return. A conditional blr does not map well to most compiled languages. It is nice for hand-written machine code though :-) For compiled code you almost always want to make some of the associated stack frame maintenance conditional as well (because the ABIs require this, if nothing else!) I guess we could add some special-case stuff for it, maybe a peephole, but we probably would have to write a whole new pass for it, stuff with branch instructions always requires special checks :-p > That way we do the return checks, but don't terminate the control flow. > After all, when the condition is taken, we had better have the stack > frame in the same state etc. Ugh, mixed abstractions again :-( Segher