Re: s390 qemu boot failure in -next

[Cornelia Huck]

On Mon, 25 Jun 2018 09:27:59 +0200
Christian Borntraeger <borntraeger@xxxxxxxxxx> 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?

> > 
> > 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?

>  
>          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...