From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 33B2C354ADD; Tue, 28 Oct 2025 22:01:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761688872; cv=none; b=OpG/qO347d31g5/jcAjls1qMPXRSGpJMVpUqY/74W3o/RpzybZLeL6L6IUq8ey3slSpr7Rt158YxQNN+b3gkiBi9on72RVgxMlwhDUq164TU7iYRzDbPvfpVXdBL4qARA74McXMPDYpDQPVorNilwf/jag9awaz9Pfn0GC6cPFA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761688872; c=relaxed/simple; bh=axkFG9sBBk92SZMcb2CWNRulpTDXD1Y8N5Mk/ScHrDk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=udku8fTep8S78FvXH6P1G0ZE5uFRGVdMll0Z6dxM20hVDlUhRZJgBPYrV0wu5OgzEt3TNmfrp95S7lKP5Va4DtAD+TAYGqtmlEpcH5JcGZQ8NJI88OZlKSci5U4mxT9GLTsMk01mcgidie9rA+lwA1OHXGMdI/abgwAfszGLKak= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=QM0AKc5D; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="QM0AKc5D" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 07A86C4CEE7; Tue, 28 Oct 2025 22:01:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1761688871; bh=axkFG9sBBk92SZMcb2CWNRulpTDXD1Y8N5Mk/ScHrDk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=QM0AKc5DCU3iBAaMAaoULokkQsILBOAEqZhala/2ILEwKDeRFwKfN9E2ZmXf+ClUt qWcVJfO6x6GOsDKqVD23u1SIhKoHzgputa6M4WzjV7S2YnOy3w7Hqpm8agAnf2zrxE PkcxM0zFoHY1EC4SD2JFlQdf+g8TlGXAdoWkv/H4AOiVt6MTmz+BGoOS4NSifJpcQW a2wcNWebFB+7QJJe/sZ5znEkG17uL8Rk3q7kCUs6WJ4VSaX27iCQbqKox/IToLeP+w T0gTgUj/FYKWhneLLpKO8ZHwhBA0ZBTBd/UxpvT2SnX727l2ky+QE0R0NecZyiUGez PEXDI4rQ41Heg== Date: Tue, 28 Oct 2025 15:01:05 -0700 From: Nathan Chancellor To: Alexander Lobakin Cc: Kees Cook , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Nick Desaulniers , Bill Wendling , Justin Stitt , Sami Tolvanen , Russell King , Tony Nguyen , Michal Kubiak , linux-kernel@vger.kernel.org, llvm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, intel-wired-lan@lists.osuosl.org Subject: Re: [PATCH 3/3] libeth: xdp: Disable generic kCFI pass for libeth_xdp_tx_xmit_bulk() Message-ID: <20251028220105.GC1548965@ax162> References: <20251025-idpf-fix-arm-kcfi-build-error-v1-0-ec57221153ae@kernel.org> <20251025-idpf-fix-arm-kcfi-build-error-v1-3-ec57221153ae@kernel.org> <20251027205409.GB3183341@ax162> <5eb7ba26-8ecb-4a39-b9ed-961fffe4aa97@intel.com> Precedence: bulk X-Mailing-List: netdev@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: <5eb7ba26-8ecb-4a39-b9ed-961fffe4aa97@intel.com> On Tue, Oct 28, 2025 at 05:29:30PM +0100, Alexander Lobakin wrote: > From: Nathan Chancellor > Date: Mon, 27 Oct 2025 13:54:09 -0700 > > > On Mon, Oct 27, 2025 at 03:59:51PM +0100, Alexander Lobakin wrote: > >> Hmmm, > >> > >> For this patch: > >> > >> Acked-by: Alexander Lobakin > > > > Thanks a lot for taking a look, even if it seems like we might not > > actually go the route of working around this. > > > >> However, > >> > >> The XSk metadata infra in the kernel relies on that when we call > >> xsk_tx_metadata_request(), we pass a static const struct with our > >> callbacks and then the compiler makes all these calls direct. > >> This is not limited to libeth (although I realize that it triggered > >> this build failure due to the way how I pass these callbacks), every > >> driver which implements XSk Tx metadata and calls > >> xsk_tx_metadata_request() relies on that these calls will be direct, > >> otherwise there'll be such performance penalty that is unacceptable > >> for XSk speeds. > > > > Hmmmm, I am not really sure how you could guarantee that these calls are > > turned direct from indirect aside from placing compile time assertions > > around like this... when you say "there'll be such performance penalty > > You mean in case of CFI or in general? Because currently on both GCC and > Clang with both OPTIMIZE_FOR_{SIZE,SPEED} they get inlined in every driver. I mean in general but obviously that sort of optimization is high value for the compiler to perform so I would only expect it not to occur in extreme cases like sanitizers being enabled; I would expect no issues when using a backend CFI implementation > > that is unacceptable for XSk speeds", does that mean that everything > > will function correctly but slower than expected or does the lack of > > proper speed result in functionality degredation? > > Nothing would break, just work way slower than expected. > xsk_tx_metadata_request() is called for each Tx packet (when Tx metadata > is enabled). Average XSK Tx perf is ~35-40 Mpps (millions of packets per > second), often [much] higher. Having an indirect call there would divide > it by n. Ah okay. > >> Maybe xsk_tx_metadata_request() should be __nocfi as well? Or all > >> the callers of it? > > > > I would only expect __nocfi_generic to be useful for avoiding a problem > > such as this. __nocfi would be too big of a hammer because it would > > Yep, sorry, I actually meant __nocfi_generic... I figured, just wanted to make sure! This series needs to go to mainline sooner rather than later, so maybe xsk_tx_metadata_request() could pick up __nocfi_generic as a future change to net-next since there is no obvious breakage? 32-bit ARM is the only architecture affected by this change since all other architectures that support kCFI have a backend specific lowering and I am guessing very few people would actually notice this problem in practice. Thanks again for chiming in and taking a look! Cheers, Nathan