Re: [Qemu-devel] [PATCH v2 02/26] armv7m: Undo armv7m.hack

[Peter Maydell <peter.maydell@linaro.org>]

On 28 December 2015 at 01:55, Michael Davidsaver <mdavidsaver@gmail.com> wrote:
> On 12/17/2015 10:38 AM, Peter Maydell wrote:
>> We could use a comment here (a) explaining what we're doing and (b)
>> mentioning that this isn't architecturally correct -- ideally we should
>> catch these exception exits on execution of the jump insn, not by
>> letting the jump execute and then trapping when we actually try to
>> execute at the magic addresses.
>
> I had an instructive little digression to investigate doing things the
> "right way" (in tcg).  I can see how it would be done by adding a
> conditional every time the PC could be updated.  To me the unassigned
> handler trick/hack seems simpler (less likely to add a bug) and avoids
> emitting more code for every ldm/pop instruction.

Yes, it's faster, which is why we do it this way. It is however
not what the hardware does (in a way which is visible to guest code
which is specifically looking for the difference), which is why it's
worth commenting on.

thanks
-- PMM