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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E1965C433ED for ; Fri, 23 Apr 2021 01:35:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9C1886144D for ; Fri, 23 Apr 2021 01:35:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239954AbhDWBgZ (ORCPT ); Thu, 22 Apr 2021 21:36:25 -0400 Received: from mga17.intel.com ([192.55.52.151]:55629 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231261AbhDWBgX (ORCPT ); Thu, 22 Apr 2021 21:36:23 -0400 IronPort-SDR: 0GaWCl+tLFAJLWt1E/kAOGlacWOYEMF+6Ke6WL70B72fsVS9UMcKMpYSKSvwSEX5Ve8T5T7UWr D658FZf+BHbQ== X-IronPort-AV: E=McAfee;i="6200,9189,9962"; a="176123785" X-IronPort-AV: E=Sophos;i="5.82,244,1613462400"; d="scan'208";a="176123785" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by fmsmga107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Apr 2021 18:35:48 -0700 IronPort-SDR: 7COMgSPIn92wJ8pf4KhIGkThc41EHZ/MG5ENrO7WxsLlgZ/+7goY2y+DkvdDz0Jx/rUswQDV3t +zktEf4d18zQ== X-IronPort-AV: E=Sophos;i="5.82,244,1613462400"; d="scan'208";a="428195313" Received: from tassilo.jf.intel.com ([10.54.74.11]) by orsmga008-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Apr 2021 18:35:47 -0700 Date: Thu, 22 Apr 2021 18:35:46 -0700 From: Andi Kleen To: Dave Hansen Cc: "Kuppuswamy, Sathyanarayanan" , Peter Zijlstra , Andy Lutomirski , Kirill Shutemov , Kuppuswamy Sathyanarayanan , Dan Williams , Raj Ashok , Sean Christopherson , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/1] x86/tdx: Add __tdcall() and __tdvmcall() helper functions Message-ID: <20210423013546.GK1401198@tassilo.jf.intel.com> References: <8723950c-e07c-9a03-503a-ab232701d1e9@linux.intel.com> <77a13ae9-0220-030e-7ae4-fd26edd7b110@intel.com> <2a3f6b3d-cd80-0734-ce83-c067666c8326@linux.intel.com> <14332abf-c78c-3bc2-9a7c-ceacfa7a0661@intel.com> <596175e3-9d1e-6c9c-fadb-ad02c396e3ad@linux.intel.com> <9161efc0-fd25-d239-32b7-5d2c726579b0@linux.intel.com> <4ac4ed35-212b-f7ad-55f4-937946ffec1a@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4ac4ed35-212b-f7ad-55f4-937946ffec1a@intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 22, 2021 at 06:21:07PM -0700, Dave Hansen wrote: > On 4/22/21 6:09 PM, Kuppuswamy, Sathyanarayanan wrote: > > But let me try to explain it here. What I meant by complication is, > > for in/out instruction, we use alternative_io() to substitute in/out > > instructions with tdg_in()/tdg_out() assembly calls. So we have to ensure > > that we don't corrupt registers or stack from the substituted instructions > > > > If you check the implementation of tdg_in()/tdg_out(), you will notice > > that we have added code to preserve the caller registers. So, if we use > > C wrapper for this use case, there is a chance that it might mess > > the caller registers or stack. > > > >     alternative_io("in" #bwl " %w2, %" #bw "0",            \ > >             "call tdg_in" #bwl, X86_FEATURE_TDX_GUEST,    \ > >             "=a"(value), "d"(port)) > > Are you saying that calling C functions from inline assembly might > corrupt the stack or registers? Are you suggesting that you simply It's possible, but you would need to mark a lot more registers clobbered (the x86-64 ABI allows to clobber many registers) I don't think the stack would be messed up, but there might be problems with writing the correct unwind information (which tends to be tricky) Usually it's better to avoid it. -Andi > can't call C functions from inline assembly? Or, that you can't express > the register clobbers of a function call in inline assembly? > > You might want to check around the kernel to see how other folks do it.