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 <[email protected]> Alex
