Re: Bug report: kernel paniced while booting

From: Alexandre Ghiti
Date: Tue Jun 06 2023 - 03:21:17 EST


On 05/06/2023 22:55, Atish Patra wrote:
On Mon, Jun 5, 2023 at 8:13 AM Sunil V L <sunilvl@xxxxxxxxxxxxxxxx> wrote:
On Mon, Jun 05, 2023 at 04:25:06PM +0200, Alexandre Ghiti wrote:
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]

@Alex: The OpenSBI region is marked reserved because EDK2 chooses to
do that explicitly as explained by Sunil.
I don't think UC has to do anything with it. All the EFI memory regions are UC.


Oops, thanks for correcting me, UC has nothing to do with that indeed, I misread is_usable_memory() (https://elixir.bootlin.com/linux/latest/source/drivers/firmware/efi/efi-init.c#L127): EFI_RESERVED_TYPE will return false.

And then this region won't get mapped, so that's the equivalent of "no-map".



- 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.

Hi Alex,

EDK2 marks opensbi range as reserved memory in EFI map. According to DT
spec, if the no-map is not set, we need to mark it as
EfiBootServicesData but EfiBootServicesData is actually considered as
free memory in kernel, as per UEFI spec. To avoid kernel using this
memory, we deviated from the DT spec for opensbi ranges.

IMO, that should be the correct way unless we can change it to
EfiRunServicesData/Code.


I'd say that everything depends on "reusable" no?

If "no-map" then
   EfiReserved
else
   if reusable then
      EfiBootServicesData
   else
      EfiRuntimeServicesData
   endif
endif

For now, I think only the following is done (based on "3.5.4 /reserved-memory and UEFI" of the DT spec https://github.com/devicetree-org/devicetree-specification/releases/download/v0.4-rc1/devicetree-specification-v0.4-rc1.pdf):

If "no-map" then
   EfiReserved
else
   EfiBootServicesData
endif


Looking at U-Boot code, it sets the no-map region to
EfiBootServicesData which may cause
issues in RISC-V as well if we linear mapping sets up the initial 2MB.


Hmm https://elixir.bootlin.com/u-boot/v2023.07-rc3/source/lib/efi_loader/efi_dt_fixup.c#L25 seems to follow what I quoted from the DT spec above:

    if (nomap)
        type = EFI_RESERVED_MEMORY_TYPE;
    else
        type = EFI_BOOT_SERVICES_DATA;


And I don't find anything regarding "reusable" in u-boot code.




Let me know your thoughts how we can handle this better in EDK2
considering it has to support ACPI also.

Thanks,
Sunil

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