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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 22AF4C433EF for ; Fri, 12 Nov 2021 16:17:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ECD2E60EE2 for ; Fri, 12 Nov 2021 16:17:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235330AbhKLQUo (ORCPT ); Fri, 12 Nov 2021 11:20:44 -0500 Received: from mga03.intel.com ([134.134.136.65]:37882 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233793AbhKLQUn (ORCPT ); Fri, 12 Nov 2021 11:20:43 -0500 X-IronPort-AV: E=McAfee;i="6200,9189,10166"; a="233100337" X-IronPort-AV: E=Sophos;i="5.87,229,1631602800"; d="scan'208";a="233100337" Received: from orsmga005.jf.intel.com ([10.7.209.41]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 Nov 2021 08:17:52 -0800 X-IronPort-AV: E=Sophos;i="5.87,229,1631602800"; d="scan'208";a="670694016" Received: from harshilp-mobl.amr.corp.intel.com (HELO skuppusw-desk1.amr.corp.intel.com) ([10.254.57.123]) by orsmga005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 Nov 2021 08:17:51 -0800 Subject: Re: [PATCH v8 08/11] x86/tdx: Wire up KVM hypercalls To: Sean Christopherson , "Yamahata, Isaku" Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, Paolo Bonzini , David Hildenbrand , Andrea Arcangeli , Josh Poimboeuf , Juergen Gross , Deep Shah , VMware Inc , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Peter H Anvin , Dave Hansen , Tony Luck , Dan Williams , Andi Kleen , Kirill Shutemov , Kuppuswamy Sathyanarayanan , linux-kernel@vger.kernel.org References: <20211005025205.1784480-1-sathyanarayanan.kuppuswamy@linux.intel.com> <20211005025205.1784480-9-sathyanarayanan.kuppuswamy@linux.intel.com> From: Sathyanarayanan Kuppuswamy Message-ID: Date: Fri, 12 Nov 2021 08:17:50 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org +Isaku Hi Isaku/Kirill, On 11/5/21 1:59 PM, Sean Christopherson wrote: > Why use a magic string? There are already mechanisms for the host to announce > itself to the guest, i.e. the guest shouldn't be attempting these hypercalls unless > it knows it's running on KVM (or something that implements KVM's ABI, whatever > that may be). > > The only use case I can think of is to support multiple flavors of hypercalls in > the VMM, e.g. to let KVM support both KVM and Hyper-V hypercalls when KVM is > masquerading as Hyper-V, but this magic value alone isn't sufficient. > > And if there is a future where KVM wants to support multiple sets of hypercalls, > using the entire 64-bit GPR for a magic value is unnecessary and wasteful, e.g. > it requires an overside MOV imm, reg. > > Why not use a single high bit? Actually, looking at KVM's set of hypercalls, > the guest can literally pass @nr as is. The GHCI defines all non-zero values as > vendor owned, i.e. this needs to ensure only that @nr is non-zero. For whatever > reason, perhaps to avoid false positives if the guest forgets to fill the value, > KVM's hypercalls start at '1'. > > Regardless of what is stuffed into r10 for the TDVMCALL, if it's anything other > than KVM's raw hypercall number, it absolutely must go into > include/uapi/linux/kvm_para.h and it should be done as a standalone commit, > because what you're proposing here is effectively committing KVM to supporting > an ABI. That is not remotely clear from the changelog. Do you see any issues with using a single bit or just passing the @nr as it is? -- Sathyanarayanan Kuppuswamy Linux Kernel Developer