Re: [Qemu-devel] [PATCH v3 04/11] linux-user: arm: set CPSR.E correctly for BE8 mode

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

On 23 June 2015 at 19:43, Peter Crosthwaite
<peter.crosthwaite@xilinx.com> wrote:
> On Tue, Jun 23, 2015 at 1:04 AM, Peter Maydell <peter.maydell@linaro.org> wrote:
>> The Linux userland ABI says:
>>  (1) the ELF file defines whether an executable is BE8 or not
>>  (2) this setting affects:
>>     (a) whether we start at the process entry point in BE or LE
>>     (b) whether we run signal handlers in BE or LE
>>     (c) whether newly cloned threads start in BE or LE
>>
>> signal_cpsr_e is how this patch implements that -- we set it
>> based on the ELF file flags, then set CPSR.E based on it:
>>  * in main, for the initial thread
>>  * in cpu_clone_regs, for subsequent threads

Aside: this is a bug in the patch which I noted first time
round with code review -- new threads don't get CPSR.E reset
like this, they inherit the CPSR.E of the thread they're
cloned from.

>>  * in signal.c, for signal handlers

This is what the flag is really for.

>> For AArch64 BE we will need something similar. I don't know if
>> there's somewhere more appropriate to store this "what's the
>> ELF file endianness" state, but we do need to keep it somewhere...
>>
>
> So my current thinking is the new state captured in TB flags,
> disas-context and this thing is just a bool for endianess. No sense of
> CPSR.E or SCTLR.xx in the newly added state across the series. The TB
> flag is then based on SCTLR.EE, SCTLR.E0E or CPSR.E depending on
> processor mode. We already have arm_cpu_is_big_endian() to calculate
> this.

I'm confused. arm_cpu_is_big_endian() tells you whether the CPU
is *currently* big-endian or not. That doesn't help you with
answering the question "I'm about to run a signal handler; what
should I set the CPSR.E bit to?" in linux-user mode. That's
what signal_cpsr_e does.

> That means that this logic would change signal_cpsr_e to a generic
> endianess bool that will set both SCTLR_EL1.E0E and CPSR.E at all the
> points Paolo is patching. SCTLR.EEs shouldn't need patching as
> usermode shouldn't be affected (maybe add an assert in
> arm_cpu_big_endian for usermode).

I'm not entirely sure what you're suggesting here, but
a "generic endianness bool" sounds more confusing than something
that's specific about exactly what it's trying to control.
"endianness for data accesses", "endianness for code accesses",
"BE8 vs BE32", "setting of TARGET_WORDS_BIGENDIAN", "endianness
to use for signal handlers", "exception endianness" and so on
are all different concepts which can't necessarily be collapsed
into a single "endianness bool".

thanks
-- PMM