Re: [Qemu-arm] [PATCH v5 00/17] target/arm: Reduce overhead of cpu_get_tb_cpu_state

[Peter Maydell]

On Wed, 21 Aug 2019 at 00:54, Richard Henderson
<address@hidden> wrote:
> However, while working on this patch set, I noticed that we have a lot of
> unnecessary overlap between A- and M- profile in the TBFLAGs.  Thus point 4
> above and the completely separate rebuild_hflags_m32().
>
> If we rearrange things like the appended, then we recover 4 bits.

You can't make the THUMB bit A-profile only: we need it in
M-profile too, so we can correctly generate code that takes
the InvalidState exception for attempts to execute with the
Thumb bit not set.

If you want to make VFPEN be A-profile only you need to
do something so we don't look at it for M-profile: currently
we set it always-1 for M-profile so we don't trip the code
that causes us to take an exception if it's 0.

Otherwise seems reasonable. My overall question is: how bad
is it if we just start using bits in the cs_base word?
If we try to get too tricky with using the same bits for
different purposes it opens the door for accidentally writing
code where we use a bit that isn't actually set correctly
for all the situations where we're using it.

thanks
-- PMM