[PATCH v4 0/5] ARM: support THREAD_INFO_IN_TASK

[Arnd Bergmann]

On Tue, Sep 14, 2021 at 11:50 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> On Tue, 14 Sept 2021 at 11:44, Arnd Bergmann <arnd at arndb.de> wrote:
>
> > I haven't looked in detail, but it all looks great to me so far.
> >
> > On a related note, this reminds me of the CONFIG_IRQSTACK series
> > that was posted last year[1] and probably attempted before that.
> > I'd have to get back to understanding all the details of that discussion,
> > but I have some hope that adding irqstacks would become much
> > easier after your series. Any thoughts on that?
>
> Yes, not having to rely on the stack pointer for thread_info/current
> seems to make this a lot easier. And eith mentioned that he is also
> interested in vmap'ed stacks for ARM, so I have a suspicion that we
> will end up picking up some IRQ stack changes in the process as well.

Ah, perfect! Adding vmap stack also solves ones of the issues for
the 4G:4G split: since vmalloc/vmap space is always mapped in
kernel mode, copy_{from,to}_user() can then just use memcpy()
with TTBR0 switched to used memory if the kernel buffer is on the
stack, or use an intermediate buffer on the stack if it's not.

     Arnd