From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from list by lists.gnu.org with archive (Exim 4.71) id 1aaMna-0004iT-PA for mharc-grub-devel@gnu.org; Mon, 29 Feb 2016 07:19:38 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37768) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aaMnY-0004fI-9X for grub-devel@gnu.org; Mon, 29 Feb 2016 07:19:37 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aaMnV-0003tr-4r for grub-devel@gnu.org; Mon, 29 Feb 2016 07:19:36 -0500 Received: from mx2.suse.de ([195.135.220.15]:59654) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aaMnU-0003tn-QR for grub-devel@gnu.org; Mon, 29 Feb 2016 07:19:33 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 968A3ABB4; Mon, 29 Feb 2016 12:19:31 +0000 (UTC) Subject: Re: [PATCH v4 10/11] xen: modify page table construction To: The development of GNU GRUB , Daniel Kiper References: <1456120999-5639-1-git-send-email-jgross@suse.com> <1456120999-5639-11-git-send-email-jgross@suse.com> <20160222091748.GO3482@olila.local.net-space.pl> <56CAD4E0.80304@suse.com> <20160222121858.GQ3482@olila.local.net-space.pl> <56CAFF66.1080709@suse.com> <20160222124805.GS3482@olila.local.net-space.pl> <56CB09AE.3040301@suse.com> <56CF4903.6020304@gmail.com> <56D40BB6.6090904@suse.com> From: Juergen Gross Message-ID: <56D4374F.5050209@suse.com> Date: Mon, 29 Feb 2016 13:19:27 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: <56D40BB6.6090904@suse.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x (no timestamps) [generic] X-Received-From: 195.135.220.15 Cc: phcoder@gmail.com, mchang@suse.com, xen-devel@lists.xen.org X-BeenThere: grub-devel@gnu.org X-Mailman-Version: 2.1.14 Precedence: list Reply-To: The development of GNU GRUB List-Id: The development of GNU GRUB List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 29 Feb 2016 12:19:37 -0000 On 29/02/16 10:13, Juergen Gross wrote: > On 25/02/16 19:33, Andrei Borzenkov wrote: >> 22.02.2016 16:14, Juergen Gross =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >>> On 22/02/16 13:48, Daniel Kiper wrote: >>>> On Mon, Feb 22, 2016 at 01:30:30PM +0100, Juergen Gross wrote: >>>>> On 22/02/16 13:18, Daniel Kiper wrote: >>>>>> On Mon, Feb 22, 2016 at 10:29:04AM +0100, Juergen Gross wrote: >>>>>>> On 22/02/16 10:17, Daniel Kiper wrote: >>>>>>>> On Mon, Feb 22, 2016 at 07:03:18AM +0100, Juergen Gross wrote: >>>>>>>>> diff --git a/grub-core/lib/xen/relocator.c b/grub-core/lib/xen/= relocator.c >>>>>>>>> index 8f427d3..a05b253 100644 >>>>>>>>> --- a/grub-core/lib/xen/relocator.c >>>>>>>>> +++ b/grub-core/lib/xen/relocator.c >>>>>>>>> @@ -29,6 +29,11 @@ >>>>>>>>> >>>>>>>>> typedef grub_addr_t grub_xen_reg_t; >>>>>>>>> >>>>>>>>> +struct grub_relocator_xen_paging_area { >>>>>>>>> + grub_xen_reg_t start; >>>>>>>>> + grub_xen_reg_t size; >>>>>>>>> +}; >>>>>>>>> + >>>>>>>> >>>>>>>> ... this should have GRUB_PACKED because compiler may >>>>>>>> add padding to align size member. >>>>>>> >>>>>>> Why would the compiler add padding to a structure containing two = items >>>>>>> of the same type? I don't think the C standard would allow this. >>>>>>> >>>>>>> grub_xen_reg_t is either unsigned (32 bit) or unsigned long (64 b= it). >>>>>>> There is no way this could require any padding. >>>>>> >>>>>> You are right but we should add this here just in case. >>>>> >>>>> Sorry, I don't think this makes any sense. The C standard is very c= lear >>>>> in this case: a type requiring a special alignment has always a len= gth >>>>> being a multiple of that alignment. Otherwise arrays wouldn't work. >>>> >>>> Sorry, I am not sure what do you mean by that. >>> >>> The size of any C type (no matter whether it is an integral type like >>> "int" or a structure) has always the same alignment restriction as th= e >>> type itself. So a type requiring 8 byte alignment will always have a >>> size of a multiple of 8 bytes. This is mandatory for arrays to work, = as >>> otherwise either the elements wouldn't be placed consecutively in mem= ory >>> or the alignment restrictions wouldn't be obeyed for all elements. >>> >> >> I too not follow how it is relevant to this case. We talk about intern= al >> padding between structure members, not between array elements. >> >>> For our case it means that two structure elements of the same type wi= ll >>> never require a padding between them, thus the annotation with "packe= d" >>> can't serve any purpose. >>> >> >> Well, I am not aware of any requirement. Compiler may add arbitrary >> padding between structure elements; it is only prohibited to add paddi= ng >> at the beginning. Sure, it would be unusual, but never say "never" ... >> also should Xen ever be ported to architecture where types are not >> self-aligned it will become an issue. >=20 > So you are telling me that _all_ interfaces between e.g. Linux, grub2, > Xen and all wire protocols not attributed with "packed" are just wrong? >=20 > Sorry, I don't think this is true. Okay, just found a reference: The x86 ABI states: Aggregates and Unions --------------------- Structures and unions assume the alignment of their most strictly aligned component. Each member is assigned to the lowest available offset with the appropriate alignment. The size of any object is always a multiple of the object=E2=80=98s alignment. I don't think any x86 C-compiler will violate the x86 ABI. Juergen