Re: [Qemu-devel] big endian arm.

[Peter Maydell <peter.maydell@linaro.org>]

On 11 August 2017 at 10:59, KONRAD Frederic <frederic.konrad@adacore.com> wrote:
> Hi Peters,
>
> I got some strange results since this commit:
>
> commit 9776f636455b6f0d9c14dce112242ed653f954b4
> Author: Peter Crosthwaite <crosthwaitepeter@gmail.com>
> Date:   Fri Mar 4 11:30:21 2016 +0000
>
>     arm: boot: Support big-endian elfs
>
>     Support ARM big-endian ELF files in system-mode emulation. When loading
>     an elf, determine the endianness mode expected by the elf, and set the
>     relevant CPU state accordingly.
>
>     With this, big-endian modes are now fully supported via system-mode LE,
>     so there is no need to restrict the elf loading to the TARGET
>     endianness so the ifdeffery on TARGET_WORDS_BIGENDIAN goes away.
>
>     Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
>     Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
>     [PMM: fix typo in comments]
>     Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
>
> It seems that when I try to load a big endian image on a
> Cortex-R5 it gets confused:
>  * the instructions are fine it executes some code.
>  * GDB address / insns are wrong endianness.
>  * in monitor x command is good endianness.
>  * the data order seems to be wrong endianness:
>    Here is my Hello World: lleHlroW

R profile bigendian is weird (ie not like A profile) because
it has a special fudge: SCTLR.IE is an Instruction Endianness
bit which lets you specify big-endian instruction order (IMPDEF
how it's set, but for the Cortex-R5 it's an external config
signal so the SoC can hardwire it as it likes). For A profile
this bit is always 0 and instructions are LE always. We
don't implement letting the board model set SCTLR.IE (or
the code needed to honour it being non-zero).

If your code requires SCTLR.IE to be 1 then you'll need
to implement that handling (if the instructions are being
correctly executed then that suggests it doesn't, but
you should check).

> We used to add a armeb-softmmu/qemu-system-armeb target for the
> big endian cpu but it doesn't work anymore as it's double
> reversed by the elf endianness (data_swab <= 2).

This was never the right way to do system BE :-)  We should
fix up whatever the R profile issues you see are...

> And I'm surprised we don't set the CPSR_E and SCTLR_EE bits
> accordingly?

Setting SCTLR_EE is the job of the board model: a BE board
model sets the 'cfgend' property on the CPU, which configures
the reset SCTLR.EE (or SCTLR.B for v5/v6 cores). (You may
also be able to set -cpu=something,cfgend=true on the command
line but that's kind of a hack.) SCTLR.EE is then supposed
to set the CPSR.E on exception entry including reset.

There is I think a bug here though: we set CPSR.E on
exception entry in arm_cpu_do_interrupt_aarch32():

    /* Set new mode endianness */
    env->uncached_cpsr &= ~CPSR_E;
    if (env->cp15.sctlr_el[arm_current_el(env)] & SCTLR_EE) {
        env->uncached_cpsr |= CPSR_E;
    }

but we forgot to do anything in arm_cpu_reset() to get
the CPSR.E bit right on initial reset.

That said, if you specify a BE elf file then we do
set the SCTLR.EE and CPSR.E bits on reset in do_cpu_reset()
(a change added in the commit you quote), which is probably
why we haven't noticed the arm_cpu_reset() bug.

Dunno about the gdbstub stuff.

thanks
-- PMM