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 _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
