Re: Bug report: kernel paniced while booting

[Alexandre Ghiti]

Hi Song,

On Mon, Jun 5, 2023 at 12:52 PM Song Shuai <songshuaishuai@xxxxxxxxxxx> wrote:
>
> Description of problem:
>
> Booting Linux With RiscVVirtQemu edk2 firmware, a Store/AMO page fault was trapped to trigger a kernel panic.
> The entire log has been posted at this link : https://termbin.com/nga4.
>
> You can reproduce it with the following step :
>
> 1. prepare the environment with
>    - Qemu-virt:  v8.0.0 (with OpenSbi v1.2)
>    - edk2 : at commit (2bc8545883 "UefiCpuPkg/CpuPageTableLib: Reduce the number of random tests")
>    - Linux : v6.4-rc1 and later version
>
> 2. start the Qemu virt board
>
> ```sh
> $ cat ~/8_riscv/start_latest.sh
> #!/bin/bash
> /home/song/8_riscv/3_acpi/qemu/ooo/usr/local/bin/qemu-system-riscv64 \
>         -s -nographic -drive file=/home/song/8_riscv/3_acpi/Build_virt/RiscVVirtQemu/RELEASE_GCC5/FV/RISCV_VIRT.fd,if=pflash,format=raw,unit=1 \
>         -machine virt,acpi=off -smp 2 -m 2G \
>         -kernel /home/song/9_linux/linux/00_rv_def/arch/riscv/boot/Image \
>         -initrd /home/song/8_riscv/3_acpi/buildroot/output/images/rootfs.ext2 \
>         -append "root=/dev/ram ro console=ttyS0 earlycon=uart8250,mmio,0x10000000 efi=debug loglevel=8 memblock=debug" ## also panic by memtest
> ```
> 3. Then you will encounter the kernel panic logged in the above link
>
> Other Information:
>
> 1. -------
>
> This report is not identical to my prior report -- "kernel paniced when system hibernates" [1], but both of them
> are closely related with the commit (3335068f8721 "riscv: Use PUD/P4D/PGD pages for the linear mapping").
>
> With this commit, hibernation is trapped with "access fault" while accessing the PMP-protected regions (mmode_resv0@80000000)
> from OpenSbi (BTW, hibernation is marked as nonportable by Conor[2]).
>
> In this report, efi_init handoffs the memory mapping from Boot Services to memblock where reserves mmode_resv0@80000000,
> so there is no "access fault" but "page fault".
>
> And reverting commit 3335068f8721 indeed fixed this panic.
>
> 2. -------
>
> As the gdb-pt-dump [3] tool shows, the PTE which covered the fault virtual address had the appropriate permission to store.
> Is there another way to trigger the "Store/AMO page fault"? Or the creation of linear mapping in commit 3335068f8721 did something wrong?
>
> ```
> (gdb) p/x $satp
> $1 = 0xa000000000081708
> (gdb) pt -satp 0xa000000000081708
>              Address :     Length   Permissions
>   0xff1bfffffea39000 :     0x1000 | W:1 X:0 R:1 S:1
>   0xff1bfffffebf9000 :     0x1000 | W:1 X:0 R:1 S:1
>   0xff1bfffffec00000 :   0x400000 | W:1 X:0 R:1 S:1
>   0xff60000000000000 :   0x1c0000 | W:1 X:0 R:1 S:1
>   0xff60000000200000 :   0xa00000 | W:0 X:0 R:1 S:1
>   0xff60000000c00000 : 0x7f000000 | W:1 X:0 R:1 S:1  // badaddr: ff6000007fdb1000
>   0xff6000007fdc0000 :    0x3d000 | W:1 X:0 R:1 S:1
>   0xff6000007ffbf000 :     0x1000 | W:1 X:0 R:1 S:1
>   0xffffffff80000000 :   0xc00000 | W:0 X:1 R:1 S:1
>   0xffffffff80c00000 :   0xa00000 | W:1 X:0 R:1 S:1
>
> ```
>
> 3. ------
>
> You can also reproduce similar panic by appending "memtest" in kernel cmdline.
> I have posted the memtest boot log at this link: https://termbin.com/1twl.
>
> Please correct me if I'm wrong.
>
> [1]: https://lore.kernel.org/linux-riscv/CAAYs2=gQvkhTeioMmqRDVGjdtNF_vhB+vm_1dHJxPNi75YDQ_Q@xxxxxxxxxxxxxx/
> [2]: https://lore.kernel.org/linux-riscv/20230526-astride-detonator-9ae120051159@wendy/
> [3]: https://github.com/martinradev/gdb-pt-dump

Thanks for the thorough report, really appreciated.

So there are multiple issues here:

- the first one is that the memory region for opensbi is marked as not
cacheable in the efi memory map, and then this region is not mapped in
the linear mapping:
[    0.000000] efi:   0x000080000000-0x00008003ffff [Reserved    |   |
 |  |  |  |  |  |  |  |   |  |  |  |UC]

- the second one (that I feel a bit ashamed of...) is that I did not
check the alignment of the virtual address when choosing the map size
in best_map_size() and then we end up trying to map a physical region
aligned on 2MB that is actually not aligned on 2MB virtually because
the opensbi region is not mapped at all.

- the possible third one is that we should not map the linear mapping
using 4K pages, this would be slow in my opinion, and I think we
should waste a bit of memory to align va and pa on a 2MB boundary.

So I'll fix the second issue, and possibly the third one, and if no
one looks into why the opensbi region is mapped in UC, I'll take a
look at edk2.

Sorry for that,

Alex