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

[Atish Patra]

On Thu, Jul 30, 2020 at 1:19 PM Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>
> Am 30. Juli 2020 22:11:39 MESZ schrieb Heinrich Schuchardt <xypron.glpk at gmx.de>:
> >Am 30. Juli 2020 20:31:47 MESZ schrieb Atish Patra
> ><atishp at atishpatra.org>:
> >>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.
> >>
> >
> >My interest is to have an affordable hardware platform where I can test
> >U-Boot's UEFI sub-system on RISC-V.
> >

Yeah. For U-Boot UEFI subsystem verification kendryte is a good choice.
But we shouldn't try to boot Linux via grub on that platform :)

> >With 6 MiB usable RAM. (2 MiB reserved for AI) we probably won't get
> >further than running GRUB.
> >
> >Can OpenSBI be built for the Kendryte K210 SoC? What is the size of
> >OpenSBI?
>

Usually, OpenSBI firmware size is around ~100 KB.

> Yes:
> https://github.com/riscv/opensbi/tree/master/platform/kendryte/k210
>
> So we should try if we can run U-Boot with OpenSBI on the platform.
>
> >
> >Best regards
> >
> >Heinrich
> >
> >>> @Ard
> >>> How about the EFI part of the Linux kernel?
> >>>
> >>> Best regards
> >>>
> >>> Heinrich
>


-- 
Regards,
Atish