Re: [Qemu-devel] [PATCH 2/3] PPC: Allow little-endian user mode.

[Alexander Graf <agraf@suse.de>]

On 05/08/2014 02:39 PM, Peter Maydell wrote:
> On 8 May 2014 09:39, Alexander Graf <agraf@suse.de> wrote:
>> On 05/08/2014 10:26 AM, Doug Kwan wrote:
>>> This all running PPC64 little-endian in user mode if target is configured
>>> that way.  In PPC64 LE user mode we set MSR.LE during initialization.
>>> Byteswapping logic is reversed also when QEMU is running in that mode.
>>>
>>> Signed-off-by: Doug Kwan <dougkwan@google.com>
>>
>> I can't say I'm a huge fan of this patch. It allows for really tricky
>> subtile mistakes to happen. Can't we leave the target mode configured on big
>> endian?
> Unfortunately not if you care about linux-user mode:
> linux-user mode uses the target-endian setting to figure
> out if it needs to do swapping of data in all the syscall
> interfaces.
>
> If you're going to overhaul how PPC deals with endian
> dependent loads/stores, I suspect you'll end up with a
> cleaner result if you convert to the new "specify endian
> setting as part of the memory operation" TCG ops:
> So for instance rather than having:
>
>      tcg_gen_qemu_ld16u(arg1, arg2, ctx->mem_idx);
>      if (unlikely(ctx->le_mode)) {
>          tcg_gen_bswap16_tl(arg1, arg1);
>      }
>
> it would be better to do
>      TCGMemOp op = MO_UW | (ctx->le_mode ? MO_LE : MO_BE);
>      tcg_gen_qemu_ld_i32(arg1, arg2, ctx->mem_idx, op);
>
> This will work regardless of the TARGET_WORD_BIGENDIAN
> setting, since we directly ask TCG to do an LE or BE
> access, rather than doing a target-endian access and
> then swapping. (It's also more efficient if you're
> in little-endian mode on a little endian host since
> it won't swap at all rather than swapping twice.)

Good point. We could store the "default TCGMemOp mask" in ctx and then 
only add the width per operation.


Alex