Re: x86 TCG helpers clobbered registers

[Stephane Duverger]

On Sat, Dec 05, 2020 at 06:38:25AM -0600, Richard Henderson wrote:
> The difference is that the slow path is aware that there are input registers
> that are live, containing data (addrlo, addrhi, datalo, datahi), which must be
> stored into the arguments for the slow path call.  Those input registers (and
> all other call-clobbered registers) are dead *after* the slow path call.
> 
> You are injecting your filter call while those input registers are still live.
> They will be next used by the fast-path store.
> 
> That is a very significant difference.

Ok. That's why I saved REG_L1 (prepared by tlb_load) for both
st/ld_direct use, plus datalo for st_direct only. I saw datahi is only
used for MO_64 on 32bits tcg-target. And I better understand it thanks
to you.

This leads me to that simple reflection:

If we want to filter on every memory accesses, *out of the fast-path*,
the most natural place to do so would be in store_helper() and
load_helper() from accel/tcg/cputlb.c. By doing so, every target would
benefit from filtering, and even specific helpers using cpu_ldst
functions would be intercepted. No ?

For the remaining fast-path case, it could be interesting to generate
it this time at IR level (tlb_load, jne to slow_path, direct
load/store) ? Again every target would benefit from filtering without
the need for a specific fast-path implementation in
tcg/<arch>/tcg-target.c.inc

Wouldn't it be simplier than actual mem plugin implementation, which
generate fitler callback *after* load/store and has specific extra
work for tracking memory accesses performed from helpers (afaiu) ?