linux-kernel - Re: s390 qemu boot failure in -next

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Date:   Mon, 25 Jun 2018 10:29:46 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        qemu-s390x <qemu-s390x@...gnu.org>,
        qemu-devel <qemu-devel@...gnu.org>,
        Thomas Huth <thuth@...hat.com>
Subject: Re: s390 qemu boot failure in -next



On 06/25/2018 10:05 AM, Cornelia Huck wrote:
> On Mon, 25 Jun 2018 09:27:59 +0200
> Christian Borntraeger <borntraeger@...ibm.com> wrote:
> 
>> Also adding QEMU.
>>
>> On 06/25/2018 09:10 AM, Christian Borntraeger wrote:
>>>
>>>
>>> On 06/22/2018 09:47 PM, Guenter Roeck wrote:  
>>>> Hi,
>>>>
>>>> starting with commit 's390/boot: make head.S and als.c be part of the
>>>> decompressor only' in -next, s390 immages no longer boot in qemu.
>>>> As far as I can see, the reason is that the command line is no longer
>>>> passed from qemu to the kernel, which results in a panic because the
>>>> root file system can not be mounted.
>>>>
>>>> Was this change made on purpose ? If so, is there a way to get qemu
>>>> back to working ?  
>>>
>>> Certainly not on purpose.
>>>
>>> Vasily, I can reproduce this with KVM and an external kernel boot of the vmlinux file (the elf file)
>>>
>>> e.g.
>>>
>>> qemu-system-s390 -enable-kvm -nographic -kernel vmlinux -append "this string no longer is command line"
>>>
>>> The compressed image (bzImage) seems to work fine though.
>>>
>>> This seems to be an unfortunate side effect of QEMUs ways to "guess" its Linux (checking for start
>>> address 0x10000, which is no longer true for the vmlinux file). With the pure vmlinux elf file
>>> the load address is 0x100000 as there is no unpacker.
> 
> Do we consider these locations to be an exported interface, or is it
> just QEMU guessing?

It is using  what zipl has hardcoded. This works fine for the final image (with the 
unpacker), but being able to boot the elf file was just a very nice feature of QEMU.

> 
>>>
>>> Guenter, can you check if arch/s390/boot/bzImage works for you as a workaround?  
>>
>> Something like this in QEMU 
>>
>> diff --git a/hw/s390x/ipl.c b/hw/s390x/ipl.c
>> index f278036fa7..14153ce880 100644
>> --- a/hw/s390x/ipl.c
>> +++ b/hw/s390x/ipl.c
>> @@ -187,11 +187,13 @@ static void s390_ipl_realize(DeviceState *dev, Error **errp)
>>           */
>>          if (pentry == KERN_IMAGE_START || pentry == 0x800) {
>>              ipl->start_addr = KERN_IMAGE_START;
>> -            /* Overwrite parameters in the kernel image, which are "rom" */
>> -            strcpy(rom_ptr(KERN_PARM_AREA), ipl->cmdline);
>>          } else {
>>              ipl->start_addr = pentry;
>>          }
>> +       if (ipl->cmdline) {
>> +            /* If there is a command line, put it in the right place */
>> +            strcpy(rom_ptr(KERN_PARM_AREA), ipl->cmdline);
>> +       }
> 
> Check for the magic Linux string (like in the non-elf case) first?

Even that does not exists in vmlinux but only in bzImage with the latest patchset
(in next, but not upstream yet)
> 
>>  
>>          if (ipl->initrd) {
>>              ram_addr_t initrd_offset;
>>
>> would put the command line in no matter what the start address is.
> 
> I'm for putting that one in (and backporting it to qemu-stable). It's a
> bit worrying, though, that our ipl code is so fragile...

We actually have to combine this with Thomas fix (to check for rom_ptr returning
something sane). It seems that ipl->commandline is always there, so we have to
check for strlen!=0 it seems..

I mean if somebody ask for "-append something" we can certainly always write something
if there is rom/ram.