Re: [PATCH v1 01/19] target/arm: Make cpu_exclusive_high hold the high bits

[Peter Maydell]

On Thu, 23 Feb 2023 at 16:12, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 2/23/23 05:14, Peter Maydell wrote:
> > On Thu, 16 Feb 2023 at 03:09, Richard Henderson
> > <richard.henderson@linaro.org> wrote:
> >>
> >> We currently treat cpu_exclusive_high as containing the
> >> second word of LDXP, even though that word is not "high"
> >> in big-endian mode.  Swap things around so that it is.
> >>
> >> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> >> ---
> >>   target/arm/translate-a64.c | 54 ++++++++++++++++++++------------------
> >>   1 file changed, 29 insertions(+), 25 deletions(-)
> >
> > This code change looks OK as far as it goes, but the bad
> > news is that we migrate the env.exclusive_val and
> > env.exclusive_high values in the machine state. So a
> > migration from a QEMU before this change to a QEMU with
> > this change on a BE host will get confused...
>
> Oof.  Ok, I didn't *really* need this, it just seemed to make sense.  I'll 
> add some
> commentary about "high" only meaning "high" for little-endian...

The current state of affairs is arguably broken, because it
means you can't migrate a guest from a BE host to an LE host,
because the migration stream contains host-endian-dependent
data. But if it's easy to leave this sleeping dog alone
then it will save us figuring out what we would need to do
in a post-load function :-)

-- PMM