Bug report: kernel paniced while booting

Mon Jun 5 14:42:33 PDT 2023

On 5 Jun 2023, at 16:12, Sunil V L <sunilvl at ventanamicro.com> 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 at tinylab.org> 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 at 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 at 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@mail.gmail.com/
>>> [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.
>> 
> 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.

Violating specs is never the answer. Do one of:

1. Use no-map and take the performance hit
2. Exclude the memory range from /memory itself
3. Come up with a new no-access property that’s a weaker no-map
   (i.e. that allows mapping and speculative access) and uses
   EfiRuntimeServicesData in EFI land

2 feels most normal to me, personally, but all are fine.

Jess

> 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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