Re: [Qemu-devel] [PATCH RFC 0/2] s390x: Make the s390-ccw BIOS relocatable

[Christian Borntraeger <borntraeger@de.ibm.com>]

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