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

[Atish Patra]

On Thu, Jul 30, 2020 at 4:04 AM Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>
> On 30.07.20 12:16, Sean Anderson wrote:
> > 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
> >
>
> Working around the problem inside U-Boot is easy (just some memcpy()
> calls) but the GRUB image itself also makes unaligned accesses:
>
> Unhandled exception: Load address misaligned
> EPC: 000000008030b004 RA: 00000000805a4eca TVAL: 000000008030b02e
> EPC: 000000007fd7e004 RA: 0000000080017eca reloc
>
> UEFI image [0x000000008030a000:0x0000000080433fff] pc=0x1004
>
> This is what I found in "RISC-V Unprivileged ISA V20191213"
>
> "Loads  and  stores  where  the  effective  address  is  not  naturally
> aligned to the referenced datatype (i.e., on a four-byte boundary for
> 32-bit accesses, and a two-byte boundary for 16-bit accesses) have
> behavior dependent on the EEI. An EEI may guarantee that misaligned
> loads and stores are fully supported, and so the software running inside
> the execution environment will never experience a contained or fatal
> address-misaligned trap."
>
> @Leif
> Should GRUB be built with -mstrict-align for RISC-V?
>

That shouldn't be necessary. Any real board with an MMU that can boot
Linux needs
a SBI provider such as OpenSBI. OpenSBI already implements a misaligned handler.

Are we planning to support EFI booting for NoMMU platforms ? As per my
understanding
runtime services need to be mapped through kernel page tables.

> @Ard
> How about the EFI part of the Linux kernel?
>
> Best regards
>
> Heinrich



-- 
Regards,
Atish