On Sat, Jan 8, 2022 at 9:56 PM Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>
> Just wondering: In a world of ASID/PCID - does the “lazy TLB” really
> have a worthy advantage?
>
> Considering the fact that with PTI anyhow address spaces are switched
> all the time, can’t we just get rid of it?
Hmm.. That may indeed be the right thing to do.
I think arm64 already hardcodes ASID 0 to init_mm, and that kernel
threads (and the idle threads in particular) might as well just use
that. In that kind of situation, there's likely little advantage to
reusing a user address space ID, and quite possibly any potential
advantage is overshadowed by the costs.
The lazy tlb thing goes back a *looong* way, and lots of things have
changed since. Maybe it's not worth it any more.
Or maybe it's only worth it on platforms where it's free (UP, possibly
other situations - like if you have IPI and it's "free").
If I read the history correctly, it looks like PF_LAZY_TLB was
introduced in 2.3.11-pre4 or something. Back in summer of 1999. The
"active_mm" vs "mm" model came later.
Linus
