Re: [PATCH 3/3] tests/tcg/s390x: test signed vfmin/vfmax

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Wed, 2022-07-13 at 21:14 +0530, Richard Henderson wrote:
> On 7/12/22 18:02, Ilya Leoshkevich wrote:
> > > This works, of course.  It could be simpler using EXECUTE, to
> > > store
> > > just the one
> > > instruction and not worry about an executable mapped page, but I
> > > guess it doesn't matter.
> > 
> > I thought about this too, but EX/EXRL operate only on the second
> > byte,
> > and I need to modify bytes 3-5 here.
> 
> I didn't mean modify the instruction via EX, but something like
> 
>    static char minmax[6] __attribute__((aligned(2)))
>      = { xx, yy, zz, 0, 0, 0 };
> 
>    minmax[3] = m6 ...
>    minmax[4] = ...
>    minmax[5] = op;
> 
>    asm("vl %%v25,0(%1)\n"
>        "vl %%v26,0(%2)\n"
>        "ex 0,0(%3)\n"
>        "vst %%v24,0(%0)"
>        : : "a"(v1), "a"(v2), "a"(v3), "a"(minmax)
>        : "memory", "v24", "v25", "v26);
> 
> 
> r~

Nice trick!

This works in qemu, but not natively: EX target must be executable.
I'd still like to try to find a way to establish an rwx section, and
send a v2 with this improvement.

I guess we'll need to fix the access check discrepancy some day.