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
