From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53097) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YTTfV-0000fH-VS for qemu-devel@nongnu.org; Thu, 05 Mar 2015 06:10:18 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YTTfQ-0008Rt-Sd for qemu-devel@nongnu.org; Thu, 05 Mar 2015 06:10:17 -0500 Received: from cantor2.suse.de ([195.135.220.15]:51862 helo=mx2.suse.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YTTfQ-0008Ri-Le for qemu-devel@nongnu.org; Thu, 05 Mar 2015 06:10:12 -0500 Message-ID: <54F83993.6050906@suse.de> Date: Thu, 05 Mar 2015 12:10:11 +0100 From: Alexander Graf MIME-Version: 1.0 References: <1425553341-566-1-git-send-email-thuth@linux.vnet.ibm.com> In-Reply-To: <1425553341-566-1-git-send-email-thuth@linux.vnet.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH RFC 0/2] s390x: Make the s390-ccw BIOS relocatable List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Thomas Huth , qemu-devel@nongnu.org On 05.03.15 12:02, Thomas Huth wrote: > Currently, our s390-ccw.img sits at the fix address 126 MiB in memory. > This has two big disadvantages: 1) We can not start guests with less > than 128 MiB RAM and 2) if the guest uses a really huge ramdisk > 126 MiB, > the s390-ccw BIOS gets overwritten and the boot silently crashes. > > These two patches now fix these problems by relocating the s390-ccw > BIOS to the end of the RAM. The basic idea here is to compile the > BIOS with the "-fpie" compiler option to create position independent > code. Sounds easy at a first glance - however, with -fpie, we only > get position independent _code_ - and a so called GOT (global offset > table) which contains absolute references to global variables again > (this is normally needed for supporting dynamic libraries - in our > s390-ccw BIOS, it's just bad luck that we get a GOT). > > So to be able to really move around our s390-ccw.img in RAM, we've got > to relocate the entries in the GOT, too. This is what the first patch > is good for. I've changed the ELF loader there to parse the reloc section > of the ELF file. I only included the bare minimum of relocation types > (R_390_RELATIVE) in the patch so far, but this can easily be extended > in case we need more (with different compiler version etc.). > > The second patch then adds the required changes to the s390-ccw BIOS > Makefile and our s390 ipl code in QEMU. > > Now I'd like to get some feedback on this approach: Is it ok to extend > the ELF loader this way? Does anybody have better/nicer ideas to solve > the problem of a relocatable BIOS? > > Thanks for any insights! I think the approach is perfectly valid and good :) Alex