Re: mgag200 fails kdump kernel booting

From: David Airlie <airlied@redhat.com>
To: Baoquan He <bhe@redhat.com>
Cc: kexec@lists.infradead.org, x86@kernel.org,
	linux-kernel <linux-kernel@vger.kernel.org>,
	dyoung@redhat.com, Lyude Paul <lyude@redhat.com>
Subject: Re: mgag200 fails kdump kernel booting
Date: Tue, 2 Jul 2019 06:51:03 +1000	[thread overview]
Message-ID: <CAMwc25oeskFG4bbrb3rwotqi1a5z4wYsGW=Qs_XJmhX_vAdNfQ@mail.gmail.com> (raw)
In-Reply-To: <20190626082907.GY24419@MiWiFi-R3L-srv>

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.

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.

Dave.
>
> >
> > To confirm it's caused by VGA printing, I blacklist the mgag200 by
> > writting it into /etc/modprobe.d/blacklist.conf. The kdump kernel can
> > boot up successfully. And add 'nomodeset' can also make it work. So it's
> > for sure mgag driver or related code have something wrong when booting
> > code tries to re-init it.
> >
> > This is the only case we ever see, tend to pursuit fix in mgag200 driver
> > side. Any idea or suggestion? We have two machines to be able to
> > reproduce it stablly.
> >
> > Thanks
> > Baoquan