Re: [Qemu-arm] [PATCH v1 13/17] arm: linux-user: don't set CPSR.E in BE32 mode

[Peter Maydell]

On 19 January 2016 at 17:26, Peter Maydell <address@hidden> wrote:
> On 18 January 2016 at 07:12, Peter Crosthwaite
> <address@hidden> wrote:
>> Don't set CPSR.E for BE32 linux-user mode. As linux-user mode models
>> BE32, using normal BE (and system mode will not), a special case is
>> needed for user-mode where if sctlr.b is set, the CPU identifies as BE.
>>
>> Signed-off-by: Peter Crosthwaite <address@hidden>
>> ---
>>
>>  linux-user/main.c |  2 --
>>  target-arm/cpu.h  | 12 +++++++++++-
>>  2 files changed, 11 insertions(+), 3 deletions(-)
>>
>> diff --git a/linux-user/main.c b/linux-user/main.c
>> index d481458..60375fb 100644
>> --- a/linux-user/main.c
>> +++ b/linux-user/main.c
>> @@ -4496,8 +4496,6 @@ int main(int argc, char **argv, char **envp)
>>              env->uncached_cpsr |= CPSR_E;
>>          } else {
>>              env->cp15.sctlr_el[1] |= SCTLR_B;
>> -            /* We model BE32 as regular BE, so set CPSR_E */
>> -            env->uncached_cpsr |= CPSR_E;
>
> ...this is undoing what we just did in the previous patch and
> which I reviewed as being the wrong thing there...
>
>>          }
>>  #endif
>>      }
>> diff --git a/target-arm/cpu.h b/target-arm/cpu.h
>> index 3edd56b..96b1e99 100644
>> --- a/target-arm/cpu.h
>> +++ b/target-arm/cpu.h
>> @@ -1812,7 +1812,17 @@ static bool arm_cpu_is_big_endian(CPUARMState *env)
>>
>>      /* In 32bit endianness is determined by looking at CPSR's E bit */
>>      if (!is_a64(env)) {
>> -        return (env->uncached_cpsr & CPSR_E) ? 1 : 0;
>> +        return
>> +#ifdef CONFIG_USER_ONLY
>> +            /* In user mode, BE32 data accesses are just modelled as
>> +             * regular BE access. In system mode, BE32 is modelled as
>> +             * little endian, with the appropriate address translations on
>> +             * non-word accesses. So sctlr.b only affects overall
>> +             * endianness in user mode
>> +             */
>> +            arm_sctlr_b(env) ||
>> +#endif
>> +                ((env->uncached_cpsr & CPSR_E) ? 1 : 0);
>>      }
>
> This doesn't seem quite right -- for system emulation we currently
> pick MO_BE or MO_LE based on the TB flag which is set according
> to (arm_cpu_is_big_endian(env). So if we ignore SCTLR.B in
> system mode then we'll still try to do LE accesses.

Ah, no, looking at the next patch this is correct, it's just the
comment is a touch confusing. I suggest

 /* In system mode, BE32 is modelled in line with the architecture
  * (as word-invariant big-endianness), where loads and stores are done
  * little endian but from addresses which are adjusted by XORing
  * with the appropriate constant. So the endianness to use for the
  * raw data access is not affected by SCTLR.B.
  * In user mode, however, we model BE32 as byte-invariant big-endianness
  * (because user-only code cannot tell the difference), and so we
  * need to use a data access endianness that depends on SCTLR.B.
  */

thanks
-- PMM