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

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH 2/3] PPC: Allow little-endian user mode.
Date: Thu, 08 May 2014 14:44:20 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130910 Thunderbird/17.0.9

On 05/08/2014 02:39 PM, Peter Maydell wrote:
On 8 May 2014 09:39, Alexander Graf <address@hidden> 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 <address@hidden>

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
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

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.


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