[Qemu-devel] Should qemu -kernel option be able to gunzip kernel? (aarch64)

[Qemu-devel] Should qemu -kernel option be able to gunzip kernel? (aarch64)

[Richard W.M. Jones]

On Fedora/aarch64, the kernel in /boot is a gzip-compressed file:

$ file /boot/vmlinuz-*
/boot/vmlinuz-0-rescue-520be7dc677d4fab99d3d3ce91f90c84: gzip compressed data, max compression, from Unix
/boot/vmlinuz-3.16.0-0.rc6.git3.1.rwmj2.fc22.aarch64:    gzip compressed data, max compression, from Unix
/boot/vmlinuz-3.16.0-0.rc6.git3.1.rwmj3.fc22.aarch64:    gzip compressed data, max compression, from Unix
/boot/vmlinuz-3.16.0-0.rc6.git3.1.rwmj4.fc22.aarch64:    gzip compressed data, max compression, from Unix

[not just for the custom kernels I've been building ...]

Apparently UEFI and u-boot can just handle this by uncompressing the
kernel on the fly.

However qemu-system-aarch64 -kernel option definitely can*not*, with
the result that if you do the "obvious":

  qemu-system-aarch64 -kernel /boot/vmlinuz-xxx

it just hangs immediately, because it's trying to execute gzip data.

I don't know the rights and wrongs of this.  Maybe Fedora is wrong for
having a gzip-compressed kernel.  But should the -kernel option be
able to detect if the file is gzip encoded and uncompress it on the
fly?  (If so I'll post a patch for this unless someone else jumps in).

Mainly I'm asking because I've no idea what is the right thing to do
here.  There could be another obvious fix.

Rich.

-- 
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Re: [Qemu-devel] Should qemu -kernel option be able to gunzip kernel? (aarch64)

[Peter Maydell]

On 29 July 2014 20:49, Richard W.M. Jones <rjones@redhat.com> wrote:
> On Fedora/aarch64, the kernel in /boot is a gzip-compressed file

> Apparently UEFI and u-boot can just handle this by uncompressing the
> kernel on the fly.
>
> However qemu-system-aarch64 -kernel option definitely can*not*, with
> the result that if you do the "obvious":
>
>   qemu-system-aarch64 -kernel /boot/vmlinuz-xxx
>
> it just hangs immediately, because it's trying to execute gzip data.
>
> I don't know the rights and wrongs of this.  Maybe Fedora is wrong for
> having a gzip-compressed kernel.  But should the -kernel option be
> able to detect if the file is gzip encoded and uncompress it on the
> fly?  (If so I'll post a patch for this unless someone else jumps in).
>
> Mainly I'm asking because I've no idea what is the right thing to do
> here.  There could be another obvious fix.

Yeah, the AArch64 approach to compressed kernels is "the
bootloader should do it" (unlike the 32 bit ARM approach
where the kernel has a decompressor tacked on the front
of it). So I agree that we should be a good bootloader and
do the decompression, assuming it's not a total pain to do so
only for AArch64 kernels.

thanks
-- PMM