efi_loader/RISC-V: misaligned load when running grubriscv64.efi

[Sean Anderson <seanga2@gmail.com>]

On 7/30/20 6:03 AM, Heinrich Schuchardt wrote:
> Dear Sean,
> 
> when trying to run grubriscv64.efi from the
> trini/u-boot-gitlab-ci-runner:bionic-20200526-18Jun2020 Docker image on
> a MAIXDUINO the relocations are not naturally aligned:
> 
> lib/efi_loader/efi_image_loader.c(133) efi_loader_relocate():
> 
>     efi_reloc 000000008030a000, offset 0x101e, type 10
> 
> Here we are trying to change an u64 at 0x8030B01E:
> 
>     uint64_t *x64 = efi_reloc + offset;
>     *x64 += (uint64_t)delta;
> 
> This leads to an exception in function efi_loader_relocate():
> 
>     Unhandled exception: Load address misaligned
>     EPC: 00000000805a95ac RA: 00000000805a953a TVAL: 000000008030b01e
>     EPC: 000000008001c5ac RA: 000000008001c53a reloc
> 
> The GRUB image is available here:
> 
> https://gist.github.com/xypron/522a91962248e9c3888d8554cb61ad2c/raw/b959661626b38a738673a9efb2f398b2fabd5c77/grubriscv64.efi
> 
> On QEMU the GRUB image is executed without problems:
> 
> https://gitlab.denx.de/u-boot/custodians/u-boot-efi/-/jobs/132919
> 
> The UEFI specification requires for the ARM platform that unaligned
> support is enabled. This is why we have implemented function
> allow_unaligned().
> 
> On RISC-V we have not yet implemented allow_unaligned() yet. Is there a
> way to switch RISCV64 CPUs especially the Kendryte K210 into a mode
> supporting unaligned access?

AFAIK RISC-V has no requirement that un-aligned loads/stores complete. I
believe the recommended solution is to install a trap handler which
completes the un-aligned load through a series of aligned loads and then
returns back to the application. For an example of such an
implementation, check out arch/riscv/kernel/traps_misaligned.c in Linux.
This may be too complex for U-Boot, so perhaps you can simply disallow
unaligned accesses?

--Sean