Re: mgag200 fails kdump kernel booting

[Baoquan He <bhe@redhat.com>]

On 07/02/19 at 06:51am, David Airlie wrote:
> On Wed, Jun 26, 2019 at 6:29 PM Baoquan He <bhe@redhat.com> wrote:
> >
> > On 06/26/19 at 04:15pm, Baoquan He wrote:
> > > Hi Dave,
> > >
> > > We met an kdump kernel boot failure on a lenovo system. Kdump kernel
> > > failed to boot, but just reset to firmware to reboot system. And nothing
> > > is printed out.
> > >
> > > The machine is a big server, with 6T memory and many cpu, its graphic
> > > driver module is mgag200.
> > >
> > > When added 'earlyprintk=ttyS0' into kernel command line, it printed
> > > out only one line to console during kdump kernel booting:
> > >      KASLR disabled: 'nokaslr' on cmdline.
> > >
> > > Then reset to firmware to reboot system.
> > >
> > > By further code debugging, the failure happened in
> > > arch/x86/boot/compressed/misc.c, during kernel decompressing stage. It's
> > > triggered by the vga printing. As you can see, in __putstr() of
> > > arch/x86/boot/compressed/misc.c, the code checks if earlyprintk= is
> > > specified, and print out to the target. And no matter if earlyprintk= is
> > > added or not, it will print to VGA. And printing to VGA caused it to
> > > reset to firmware. That's why we see nothing when didn't specify
> > > earlyprintk=, but see only one line of printing about the 'KASLR
> > > disabled'.
> >
> > Here I mean:
> > That's why we see nothing when didn't specify earlyprintk=, but see only
> > one line of printing about the 'KASLR disabled' message when
> > earlyprintk=ttyS0 added.
> 
> Just to clarify, the original kernel is booted with mgag200 turned
> off, then kexec works, but if the original kernel loads mgag200, the
> kexec kernels resets hard when the VGA is used to write stuff out.

Thanks for looking into this, Dave.

Yeah, in fact the issue was found in kdump kernel. I haven't checked the
kexec jumping. Kexec jumping will call device_shutdown() to attempt to
shutdown all devices before jumping to the 2nd kernel. But kdump jumping
won't.

> 
> This *might* be fixable in the controlled kexec case, but having an
> mgag200 shutdown path that tries to put the gpu back into a state
> where VGA doesn't die, but for the uncontrolled kexec it'll still be a
> problem, since once the gpu is up and running and VGA is disabled, it
> doesn't expect to see anymore VGA transactions.

Yes, I see. It should have been shutdown by device_shutdown() in kexec
case. The uncontrolled case, I guess you mean the kdump case. In
kdump case, we don't call device_shutdown() before jumping because the
1st kernel has been in crashed state, we just want to switch to kdump
kernel asap. So wondering how other GPU/VGA device/driver bebahve,
currently haven't got report about them. Probably mgag200 is very new,
or we may not meet them. This issue was met on a new bought server.

Thanks
Baoquan

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