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Subject: Re: [qemu-s390x] s390 qemu boot failure in -next
From:   Christian Borntraeger <borntraeger@de.ibm.com>
To:     Guenter Roeck <linux@roeck-us.net>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc:     linux-s390@vger.kernel.org, Thomas Huth <thuth@redhat.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        linux-kernel@vger.kernel.org, qemu-devel <qemu-devel@nongnu.org>,
        qemu-s390x <qemu-s390x@nongnu.org>
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Date:   Mon, 25 Jun 2018 10:02:28 +0200
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On 06/25/2018 09:27 AM, Christian Borntraeger 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.
>>
>> 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);
> +       }
>  
>          if (ipl->initrd) {
>              ram_addr_t initrd_offset;
> 
> would put the command line in no matter what the start address is.

Ideally we would do 2 changes:
- change QEMU to add a commandline to 10480 if specified
- have a way to fix the kernel elf file to still boot with older QEMUs