Re: [PATCH 1/1] target/i386: Raise #GP on unaligned m128 accesses when required.

[Ricky Zhou]

On Mon, Aug 29, 2022 at 9:45 AM Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 8/29/22 07:23, Ricky Zhou wrote:
> This trap should be raised via the memory operation:
> ...
> Only the first of the two loads/stores must be aligned, as the other is known 
> to be +8.
> You then must fill in the x86_tcg_ops.do_unaligned_access hook to raise #GP.
Thanks for taking a look at this - did you see the bit in the cover
letter where I discuss doing this via alignment requirements on the
memory operation? My logic was that the memop alignment checks seem to
be more oriented towards triggering #AC exceptions (even though this is
not currently implemented), since qemu-user's unaligned access handlers
(helper_unaligned_{ld,st}) already trigger SIGBUS as opposed to SIGSEGV.
I was concerned that implementing this via MO_ALIGN_16 would get in the
way of a hypothetical future implementation of the AC flag, since
do_unaligned_access would need to raise #AC instead of #GP for that.

One slightly more involved way to use alignment on the MemOp could be to
arrange to pass the problematic MemOp to do_unaligned_access and
helper_unaligned_{ld,st}. Then we could allow CPUs to handle
misalignment of different MemOps differently (e.g. raise #GP/SIGSEGV for
certain ops and #AC/SIGBUS for others). For this change to x86, we could
maybe get away with making MO_ALIGN_16 and above trigger #GP/SIGSEGV and
everything else trigger #AC/SIGBUS. If that's a little hacky, we could
instead add some dedicated bits to MemOp that distinguish different
types of unaligned accesses.

What do you think? Happy to implement whichever approach is preferred!

Thanks,
Ricky