From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54585) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YTVDb-0005Hb-Db for qemu-devel@nongnu.org; Thu, 05 Mar 2015 07:49:36 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YTVDW-0005n9-Ca for qemu-devel@nongnu.org; Thu, 05 Mar 2015 07:49:35 -0500 Received: from cantor2.suse.de ([195.135.220.15]:56725 helo=mx2.suse.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YTVDW-0005n1-6d for qemu-devel@nongnu.org; Thu, 05 Mar 2015 07:49:30 -0500 Message-ID: <54F850D8.3030805@suse.de> Date: Thu, 05 Mar 2015 13:49:28 +0100 From: Alexander Graf MIME-Version: 1.0 References: <1425553341-566-1-git-send-email-thuth@linux.vnet.ibm.com> <54F83993.6050906@suse.de> <54F84F70.8070006@de.ibm.com> In-Reply-To: <54F84F70.8070006@de.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: Christian Borntraeger , Thomas Huth , qemu-devel@nongnu.org, Paolo Bonzini On 05.03.15 13:43, Christian Borntraeger wrote: > Am 05.03.2015 um 12:10 schrieb Alexander Graf: >> >> >> 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 :) > > Yes, looks good. It relocated much higher, but not beyond 2GB, and it can > now handle guests < 128MB. > > I can take these patches via the s390 tree. > > Would be good if somebody else (Paolo,Alex ??) > could Ack the change in include/hw/elf_ops.h. Acked-by: Alexander Graf Alex