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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 62D56C83004 for ; Wed, 29 Apr 2020 14:15:13 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 33D6420B80 for ; Wed, 29 Apr 2020 14:15:13 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=xen.org header.i=@xen.org header.b="4oI9G1nH" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 33D6420B80 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=xen.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jTnUL-000671-Du; Wed, 29 Apr 2020 14:15:01 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jTnUK-00066w-64 for xen-devel@lists.xenproject.org; Wed, 29 Apr 2020 14:15:00 +0000 X-Inumbo-ID: cf62daa4-8a23-11ea-b9cf-bc764e2007e4 Received: from mail.xenproject.org (unknown [104.130.215.37]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id cf62daa4-8a23-11ea-b9cf-bc764e2007e4; Wed, 29 Apr 2020 14:14:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=xen.org; s=20200302mail; h=Content-Transfer-Encoding:Content-Type:In-Reply-To: MIME-Version:Date:Message-ID:From:References:Cc:To:Subject:Sender:Reply-To: Content-ID:Content-Description:Resent-Date:Resent-From:Resent-Sender: Resent-To:Resent-Cc:Resent-Message-ID:List-Id:List-Help:List-Unsubscribe: List-Subscribe:List-Post:List-Owner:List-Archive; bh=PVFkJQZhuj4UDnCiCNLSK8NTIxMziKd2mEPqxVB/yvI=; b=4oI9G1nHDMASB+MPmjpdIBchPR 7a3uHk59GZdjuacgXNkslBNOUTTYvabv15bhHDVLxSF0xK5X8dAHq5s1zCmS9uqNOgoIx+mg+7VGF zLiYmzEBpYM5FZrYkcdSuCcUB5zQ9BJBfbDeSkfUiDJHdqfVKvlASboIHoOnV27Qe43k=; Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jTnUH-0002tK-Lm; Wed, 29 Apr 2020 14:14:57 +0000 Received: from [54.239.6.185] (helo=a483e7b01a66.ant.amazon.com) by xenbits.xenproject.org with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.89) (envelope-from ) id 1jTnUH-0007F1-Bu; Wed, 29 Apr 2020 14:14:57 +0000 Subject: Re: [PATCH] pvcalls: Document explicitly the padding for all arches To: Jan Beulich References: <20200419104948.31200-1-julien@xen.org> <78288649-5930-9d01-bb8f-85e15406e4ef@xen.org> <6fc59120-664e-6a07-5196-57e1dbfb0dde@suse.com> <240bc5e8-f8fd-217a-fa10-7628ac9d4e6e@suse.com> <9eb39857-2e33-4a6b-1825-f9dc537a6515@xen.org> <423c0369-9c90-dbfe-2f90-d49a2ce5b283@suse.com> From: Julien Grall Message-ID: <4cd108f9-3ad0-2262-fa7c-d2247660c635@xen.org> Date: Wed, 29 Apr 2020 15:14:55 +0100 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <423c0369-9c90-dbfe-2f90-d49a2ce5b283@suse.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 8bit X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Juergen Gross , Stefano Stabellini , Wei Liu , Andrew Cooper , Julien Grall , Ian Jackson , George Dunlap , xen-devel@lists.xenproject.org Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" Hi Jan, On 29/04/2020 15:05, Jan Beulich wrote: > On 29.04.2020 16:01, Julien Grall wrote: >> Hi, >> >> On 22/04/2020 10:20, Jan Beulich wrote: >>>> Even if it was possible to use the sub-structs defined in the header >>>> that way, keep in mind that we also wrote: >>>> >>>>          /* dummy member to force sizeof(struct xen_pvcalls_request) >>>>           * to match across archs */ >>>>          struct xen_pvcalls_dummy { >>>>              uint8_t dummy[56]; >>>>          } dummy; >>> >>> This has nothing to do with how a consumer may use the structs. >>> >>>> And the spec also clarifies that the size of each specific request is >>>> always 56 bytes. >>> >>> Sure, and I didn't mean to imply that a consumer would be allowed >>> to break this requirement. Still something like this >>> >>> int pvcall_new_socket(struct xen_pvcalls_socket *s) { >>>      struct xen_pvcalls_request req = { >>>          .req_id = REQ_ID, >>>          .cmd = PVCALLS_SOCKET, >>>          .u.socket = *s, >>>      }; >>> >>>      return pvcall(&req); >>> } >>> >>> may break. >> >> I think I understand your concern now. So yes I agree this would break 32-bit consumer. >> >> As the padding is at the end of the structure, I think a 32-bit frontend and 64-bit backend (or vice-versa) should currently work without any trouble. The problem would come later if we decide to extend a command. > > Can commands be extended at all, i.e. don't extensions require new > commands? The issue I've described has nothing to do with future > extending of any of the affected structures. I think my point wasn't conveyed correctly. The implicit padding is at the end of the structure for all the consumers but 32-bit x86. So without any modification, I think 32-bit frontend can still communicate with 64-bit backend (or vice-versa). Therefore I suggest to rework the documentation and add the implicit padding just for all the architectures but 32-bit x86. Cheers, -- Julien Grall