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Date:   Tue, 2 Jul 2019 10:21:40 +0800
From:   Dave Young <dyoung@redhat.com>
To:     Baoquan He <bhe@redhat.com>
Cc:     airlied@redhat.com, kexec@lists.infradead.org, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: mgag200 fails kdump kernel booting
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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'.
> 
> 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.

Personally I think early code should not blindly do vga writing, there
are cases that does not work:
1. efi booted machine,  just no output
2. kdump kernel booted,  writing to vga caused undefined state, for
example in your case it caused a system reset.

So I suggest only write to vga when we see earlyprintk=vga in kernel
cmdline.

Thanks
Dave