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. Christian
