Re: [Qemu-devel] [PATCH] pc-bios/s390-ccw: build s390 bios with -fno-zero-initialized-in-bss

[Christian Borntraeger]

On 11/20/2017 11:24 AM, Alexander Graf wrote:
> On 11/20/2017 11:19 AM, Christian Borntraeger wrote:
>>
>> On 11/20/2017 11:16 AM, Alexander Graf wrote:
>>> On 11/20/2017 11:02 AM, Christian Borntraeger wrote:
>>>> On 11/20/2017 10:29 AM, Alexander Graf wrote:
>>>>> On 11/20/2017 10:27 AM, Christian Borntraeger wrote:
>>>>>> On 11/20/2017 10:19 AM, Alexander Graf wrote:
>>>>>>> On 11/20/2017 10:15 AM, Christian Borntraeger wrote:
>>>>>>>> The QEMU ELF loader does not initialize the bss segment. This has
>>>>>>>> triggered several bugs in the past, e.g. see commit 5d739a4787a5
>>>>>>>> ("s390-ccw.img: Fix sporadic errors with ccw boot image - initialize
>>>>>>>> css").
>>>>>>>>
>>>>>>>> Instead of fixing these things one-by-one we can build the BIOS
>>>>>>>> with -fno-zero-initialized-in-bss. This will move the zero variables
>>>>>>>> also into the data segment, which is then part of a LOAD section.
>>>>>>> Doesn't this bloat the firmware? Why don't we just manually clear bss 
>>>>>>> in the firmware itself? It's what all other firmwares do :)
>>>>>> Yes the proper fix is to initialize bss in the bios itself.
>>>>>> I was trying to come up with something for 2.11, but since the patch 
>>>>>> does not solve the original issues, lets drop it.
>>>>> Initializing bss is quite simple. You can probably even do it from C. 
>>>>> Just set two variables before and after .bss in the linker script and 
>>>>> memset(0) from start to end :).
>>>> Yes, I know but then we have to change the build process to use a linker 
>>>> script.(we rely on the default
>>>> linker script right now).
>>>
>>> I'm not sure how common generic linker scripts are, but in our default 
>>> script we have existing markers for bss and end. You can look at the 
>>> default linker script using gcc <c file> -Wl,-verbose. This simple source 
>>> worked for me:
>> Are we sure that the range between __bss_start and _end does not include 
>> other elements (besides bss)?
> 
> It seems to be the intended semantic for the linker script I see as default, 
> but I'm not an expert here :). Can you check with Uli?

I will.
But the "I am not sure" is exactly the reason why I prefer Thomas "band-aid" 
for 2.11.
Doing the bss zeroing is certainly my preferred long term solution (and
it was my first comment to Thomas' patch)